hadrian: Create relative rather than absolute symlinks in binary dist folder

The symlink structure now looks like:

```
lrwxrwxrwx 1 matt users       16 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc -> ghc-9.3.20210721
-rwxr-xr-x 1 matt users  1750336 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-9.3.20210721
lrwxrwxrwx 1 matt users       22 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-iserv -> ghc-iserv-9.3.20210721
-rwxr-xr-x 1 matt users 31703176 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-iserv-9.3.20210721
lrwxrwxrwx 1 matt users       26 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-iserv-dyn -> ghc-iserv-dyn-9.3.20210721
-rwxr-xr-x 1 matt users    40808 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-iserv-dyn-9.3.20210721
lrwxrwxrwx 1 matt users       20 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-pkg -> ghc-pkg-9.3.20210721
-rwxr-xr-x 1 matt users   634872 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/ghc-pkg-9.3.20210721
lrwxrwxrwx 1 matt users       14 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/haddock -> haddock-2.24.0
-rwxr-xr-x 1 matt users  4336664 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/haddock-2.24.0
lrwxrwxrwx 1 matt users        9 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/hp2ps -> hp2ps-0.1
-rwxr-xr-x 1 matt users    49312 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/hp2ps-0.1
lrwxrwxrwx 1 matt users        8 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/hpc -> hpc-0.68
-rwxr-xr-x 1 matt users   687896 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/hpc-0.68
lrwxrwxrwx 1 matt users       13 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/hsc2hs -> hsc2hs-0.68.8
-rwxr-xr-x 1 matt users   729904 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/hsc2hs-0.68.8
lrwxrwxrwx 1 matt users       19 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/runghc -> runghc-9.3.20210721
-rwxr-xr-x 1 matt users    57672 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/runghc-9.3.20210721
lrwxrwxrwx 1 matt users        9 Aug  3 16:27 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/unlit -> unlit-0.1
-rwxr-xr-x 1 matt users    14896 Aug  3 15:00 _build/bindist/ghc-9.3.20210721-x86_64-unknown-linux/bin/unlit-0.1
```

Fixes #20198

(cherry picked from commit e50267d9)

hadrian/src/Rules/BinaryDist.hs

@@ -14,6 +14,7 @@ import Utilities
 import Data.Either
 import Hadrian.Oracles.Cabal
 import Hadrian.Haskell.Cabal.Type
+import qualified System.Directory.Extra as IO
 
 {-
 Note [Binary distributions]
@@ -147,7 +148,11 @@ bindistRules = do
             -- a wrapper script on platforms (windows) which don't support symlinks.
             if windowsHost
               then createVersionWrapper version_prog unversioned_install_path
-              else createFileLink install_path unversioned_install_path
+              else liftIO $ do
+                -- Use the IO versions rather than createFileLink because
+                -- we need to create a relative symlink.
+                IO.removeFile unversioned_install_path <|> return ()
+                IO.createFileLink version_prog unversioned_install_path
         copyDirectory (ghcBuildDir -/- "lib") bindistFilesDir
         copyDirectory (rtsIncludeDir)         bindistFilesDir