release.sh: Allow a callback script to update version in code

[nhooyr]

Maybe instead of executing release.sh, we source it instead in the in repo scripts.

[nhooyr]

• Will also need to update tala's build process to do the same.

Perhaps just make release.sh automatically updates any lib/version/version.go? Would need to keep -HEAD suffix though unless I want the git tag to include an extra commit removing the suffix. this commit not being merged into master.

[nhooyr]

Maybe we just always update lib/version/version.go if it exists. Then no custom callback needed.