Implement 'ghcup run'

@RyanGlScott

$ ghcup run --help
Usage: ghcup run [-a|--append] [-i|--install] [--ghc GHC_VERSION]
                 [--cabal CABAL_VERSION] [--hls HLS_VERSION]
                 [--stack STACK_VERSION] [-b|--bindir DIR] [COMMAND]
  Run a command with the given tool in PATH

Available options:
  -a,--append              Append bin/ dir to PATH instead of prepending (this
                           means that e.g. a system installation may take
                           precedence)
  -i,--install             Install the tool, if missing
  --ghc GHC_VERSION        The ghc version
  --cabal CABAL_VERSION    The cabal version
  --hls HLS_VERSION        The HLS version
  --stack STACK_VERSION    The stack version
  -b,--bindir DIR          directory where to create the tool symlinks (default:
                           newly created system temp dir)
  COMMAND                  The command to run, with arguments (use longopts --).
                           If omitted, just prints the created bin/ dir to
                           stdout and exits.
  -h,--help                Show this help text

Discussion:
  Adds the given tools to a dedicated bin/ directory and adds them to PATH, exposing
  the relevant binaries, then executes a command.

Examples:
  # run VSCode with all latest toolchain exposed, installing missing versions if necessary
  ghcup run --ghc latest --cabal latest --hls latest --stack latest --install -- code Setup.hs

  # create a custom toolchain bin/ dir with GHC and cabal that can be manually added to PATH
  ghcup run --ghc 8.10.7 --cabal 3.2.0.0 --bindir $HOME/toolchain/bin

  # run a specific ghc version
  ghcup run --ghc 8.10.7 -- ghc --version

If command/arguments are omitted, it just prints the bindir (which by default is in a system temp dir) and exits. The bin dirs that are created are never cleaned up. Since they just contain symlinks, I think that's ok (ghcup gc -t can be used).

On linux, we use posix exec to spawn the process, which is cleaner.

.gitlab/script/ghcup_version.sh

@@ -97,6 +97,7 @@ eghcup --numeric-version
 
 eghcup install ghc ${GHC_VERSION}
 [ `$(eghcup whereis ghc ${GHC_VERSION}) --numeric-version` = "${GHC_VERSION}" ]
+[ `eghcup run --ghc ${GHC_VERSION} -- ghc --numeric-version` = "${GHC_VERSION}" ]
 eghcup set ghc ${GHC_VERSION}
 eghcup install cabal ${CABAL_VERSION}
 [ `$(eghcup whereis cabal ${CABAL_VERSION}) --numeric-version` = "${CABAL_VERSION}" ]
@@ -104,6 +105,22 @@ eghcup unset cabal
 "$GHCUP_BIN"/cabal --version && exit || echo yes
 eghcup set cabal ${CABAL_VERSION}
 [ `$(eghcup whereis cabal ${CABAL_VERSION}) --numeric-version` = "${CABAL_VERSION}" ]
+[ `eghcup run --cabal ${CABAL_VERSION} -- cabal --numeric-version` = "${CABAL_VERSION}" ]
+
+if [ "${OS}" != "FREEBSD" ] ; then
+	if [ "${ARCH}" = "64" ] ; then
+		eghcup run --ghc 8.10.7 --cabal 3.4.1.0 --hls 1.6.1.0 --stack 2.7.3 --install --bindir "$(pwd)/.bin"
+		if [ "${OS}" == "WINDOWS" ] ; then
+			expected=$(cat "$( cd "$(dirname "$0")" ; pwd -P )/../ghcup-run.files.windows" | sort)
+		else
+			expected=$(cat "$( cd "$(dirname "$0")" ; pwd -P )/../ghcup-run.files" | sort)
+		fi
+		actual=$(cd ".bin" && find . | sort)
+		[ "${actual}" = "${expected}" ]
+		unset actual expected
+		rm -rf .bin
+	fi
+fi
 
 cabal --version
 
@@ -133,7 +150,7 @@ else
 		eghcup --offline install ghc 8.10.3
 		if [ "${ARCH}" = "64" ] ; then
 			expected=$(cat "$( cd "$(dirname "$0")" ; pwd -P )/../ghc-8.10.3-linux.files" | sort)
-			actual=$(cd "${GHCUP_DIR}/ghc/8.10.3/" && find | sort)
+			actual=$(cd "${GHCUP_DIR}/ghc/8.10.3/" && find . | sort)
 			[ "${actual}" = "${expected}" ]
 			unset actual expected
 		fi
@@ -141,7 +158,7 @@ else
 		eghcup prefetch ghc 8.10.3
 		eghcup --offline install ghc 8.10.3
 		expected=$(cat "$( cd "$(dirname "$0")" ; pwd -P )/../ghc-8.10.3-windows.files" | sort)
-		actual=$(cd "${GHCUP_DIR}/ghc/8.10.3/" && find | sort)
+		actual=$(cd "${GHCUP_DIR}/ghc/8.10.3/" && find . | sort)
 		[ "${actual}" = "${expected}" ]
 		unset actual expected
 	else
@@ -182,6 +199,8 @@ else
 	fi
 fi
 
+
+
 # check that lazy loading works for 'whereis'
 cp "$CI_PROJECT_DIR/data/metadata/ghcup-${JSON_VERSION}.yaml" "$CI_PROJECT_DIR/data/metadata/ghcup-${JSON_VERSION}.yaml.bak"
 echo '**' > "$CI_PROJECT_DIR/data/metadata/ghcup-${JSON_VERSION}.yaml"