Makefile: Drop `release.zsh`’s filename extension.

In the Makefile, correctly refer to the release script as
`scripts/release`, not `scripts/release.zsh`.

See also this commit’s parent commit, in which the release script was
renamed from `scripts/release.zsh` to `scripts/release`:
a6c5dec0ad66e28dd6e9fc11e631087a7114edeb