Expose all supported GHC versions (#2987)

Right now the only default GHC version gets exported by the flake. This
means that libraries wanting to use the clash-compiler flake
realistically can only support the default GHC version. If someone wants
to use any of the other supported versions, that would work fine for
clash-compiler, but it wouldn't trickle down to the libraries (which are
stuck with the default ghc version).

Graphs visualizing the issue:

If you want to develop with the default version (ghc9101)
```
myprogram => ghc9101
 |- clash-compiler => ghc9101
 \- clash-protocols => ghc9101
     |- clash-compiler => ghc9101
     \- circuit-notation => ghc9101
         \- clash-compiler => ghc9101
```

With a non-default, but supported version (ghc964)
```
myprogram => ghc964
 |- clash-compiler => ghc964
 \- clash-protocols => ghc9101
     |- clash-compiler => ghc9101
     \- circuit-notation => ghc9101
         \- clash-compiler => ghc9101
```

Author: jaschutte

flake.nix

@@ -83,6 +83,10 @@
       in
       assert pkgs.lib.asserts.assertOneOf "defaultGhcVersion" defaultGhcVersion ghcVersions;
       {
+        # Export the variable so that other flakes can use it.
+        ghcVersion = defaultGhcVersion;
+        supportedGhcVersions = ghcVersions;
+
         packages = {
           inherit (pkgs."clashPackages-${defaultGhcVersion}")
             clash-benchmark