cmd/go: enable GOCACHEPROG by default

[danp]

Opening this so it can at least be considered for 1.23.

With #59719 and #60419 we got support for GOCACHEPROG in 1.21+. However, it requires GOEXPERIMENT=gocacheprog be set at toolchain build time. This means official Go builds don't support GOCACHEPROG and that can make it difficult to use.

Based on issues such as dominikh/go-tools#1458 it seems at least Tailscale are having success with GOCACHEPROG.

Should it be supported by official Go builds, removing the need for toolchain-build-time GOEXPERIMENT=gocacheprog?

[danp]

cc @bradfitz

[danp]

If this feature is not ready for non-experimental use one option would be to still require GOEXPERIMENT=gocacheprog but move it from toolchain build time to toolchain invocation time.

[rsc]

/cc @bcmills for thoughts

[bcmills]

Per the discussion in https://go.dev/cl/c/go/+/486715/14#message-086f5f820978ba6f6b2afc0589b397894afddf93, I think GOCACHEPROG needs proper integration tests in the main Go repo before we can consider promoting it to non-experimental status.

[gopherbot]

Change https://go.dev/cl/556997 mentions this issue: cmd/go: add initial cacheprog integration tests

[seankhliao]

it also needs docs, and preferably importable type definition

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group