cmd/dist: make `--compile-only` actually only compile test binaries and not run them

[mknyszek]

Currently the --compile-only flag mostly prevents tests from running, but still runs all compiled test binaries. This means that our cross-compiling builders don't have an easy way to build test packages, so we're missing some coverage there on the trybots.

CC @aclements @golang/release

[gopherbot]

Change https://go.dev/cl/464956 mentions this issue: dashboard: remove StopAfterMake from misc-compile builders

[dmitshur]

For additional context, this comment thread captures some of the reasons why -run=^$ was chosen over test -c at the time, before we considered this use case.

[dmitshur]

There were a lot of internal refactors and clean up work done as part
of being able to add proper -json flag support to cmd/dist (#37486),
and that lead up to making -compile-only flag do as its name suggests,
rather than stopping just short of that. See CL 492986 and the rest of
the CLs in the great dist test cleanup of 2023.

So this is done in Go 1.21.