crypto/x509: respect GODEBUG changes for allowing SHA1 certificates

crypto/x509: respect GODEBUG changes for allowing SHA1 certificates

This allows programs that want SHA1 support to call os.Setenv at startup
instead of insisting that users set the environment variable themselves.

For #41682.
Fixes #56436.

Change-Id: Idcb96212a1d8c560e1dd8eaf7c80b6266f16431e
Reviewed-on: https://go-review.googlesource.com/c/go/+/445496
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>