cmd/go/internal/base: rename EnvForDir to AppendPWD

EnvForDir does not immediately evoke “append”, and thus may not prompt
the reader to consider the possibility of aliasing bugs (as in
issue #38077). To make this behavior more obvious at the call site, rename
cmd/go/internal/base.EnvForDir to AppendPWD and swap the order of
arguments to a conventional “append” function (similar to those in the
strconv package).

For #38077

Change-Id: I16f09aa0fa8a269d51f0511eb402a44e2759eb94
Reviewed-on: https://go-review.googlesource.com/c/go/+/225578


Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>