cmd/go: serialize access to scriptDir with output

Currently, Builder.fmtcmd can read scriptDir without taking the output
lock. This introduces a potential data race between the read in fmtcmd
and the write in Showcmd. There's also a logical race here: because
fmtcmd doesn't know when its output is going to be printed, Showcmd
may print a "cd" command between when fmtcmd is called and when its
output is printed. As a result, it doesn't make sense to just lock
around the access in fmtcmd.

Instead, move the entire scriptDir substitution to Showcmd. This will
generally result in the same output. In the cases where Builder.run is
called with a non-empty desc, it means we may print a full path in the
comment line above output rather than substituting the script
directory. I think this is okay.

This lets us undo the workaround in CL 536355.

Change-Id: I617fe136eaafcc9bbb7e701b427d956aeab8a2b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/536376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Austin Clements <austin@google.com>

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