cmd/go: don't crash when running "go version" in deleted directory

If the go command is executed on Linux in a deleted directory,
it fails. This behavior is reasonable for commands which depend on
the CWD, but it's unexpected for commands like `go version`.
This change delays initialization of a global CWD variable.

Fixed #34499

Change-Id: I7302fb84a3b7f5f149a123d277abd5b9b5bc95b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/268261


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>