Use `BOOTSTAP_HASKELL_{GHC,CABAL}_VERSION` env vars as overrides

As discussed in ghcup-hs#30 (closed). Tested manually, works on my machine (TM) with and without the variables set

bootstrap-haskell

@@ -80,10 +80,19 @@ if [ -z "${BOOTSTRAP_HASKELL_NONINTERACTIVE}" ] ; then
     read -r answer </dev/tty
 fi
 
-eghcup --cache install
+if [ -z "${BOOTSTRAP_HASKELL_GHC_VERSION}" ] ; then
+    eghcup --cache install   
+    eghcup set
+else
+    eghcup --cache install "${BOOTSTRAP_HASKELL_GHC_VERSION}"
+    eghcup set "${BOOTSTRAP_HASKELL_GHC_VERSION}"
+fi
 
-eghcup set
-eghcup --cache install-cabal
+if [ -z "${BOOTSTRAP_HASKELL_CABAL_VERSION}" ] ; then
+  eghcup --cache install-cabal 
+else
+  eghcup --cache install-cabal "${BOOTSTRAP_HASKELL_CABAL_VERSION}"
+fi
 
 edo cabal new-update