gobash.yml: build fails for go1.15 and macos-latest

[bassosimone]

The reason why this happens is that macOS latest is now darwin/arm64 and there's no go1.15 for that GOOS/GOARCH.

Here's an excerpt from the build logs:

Run actions/setup-go@v4
Setup go version spec 1.15
Attempting to download 1.15...
matching 1.15...
Not found in manifest.  Falling back to download directly from Go
Error: Unable to find Go version '1.15' for platform darwin and architecture arm64.

The most immediate fix seems to drop macos-latest from the build. Our intent with gobash.yml is just to make sure we can build starting from old versions of Go, and the OS we are using does not actually matter much. Also, by dropping darwin builds, we also have less builds per pull request, which seems good in general.