internal/buildcfg: set Error instead of panicking

All build environment validation sets Error except for the
GOEXPERIMENT parser, which panics. Change it to also set Error so that
a bad GOEXPERIMENT doesn't cause everything that imports
internal/buildcfg to panic on init.

Change-Id: Ie9a506ef0978ecb410f2dcd784638f2167354175
Reviewed-on: https://go-review.googlesource.com/c/go/+/310970


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>