cmd/go/internal/work: make toolchain builds reproducible when buildmode=pie

When buildmode=pie, external linking is forced, and our toolchain build id
will be included in the external build id, resulting in the building of
a toolchain tool will never reach a fixed point id.

More importantly, this change will make make.bash converge on self-hosted
Android builds (Android refuses to run non-PIE executables).

Fixes #31320
Updates #18968

Change-Id: Icb5db9f4b1b688afe37f4dafe261ffda580fa4e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/170942


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>