cmd/dist: skip GOCACHE and .git when making GOROOT read-only
When we run tests, we may need to write the test binary (and/or test
variants of its dependencies) to GOCACHE. (This also fixes several
test cases in cmd/go, which preserves the GOCACHE variable for
efficiency.)
It is highly unlikely that tests will try to modify .git, and that
directory contains many files, so don't bother with it.