internal/godebug,crypto/fips140: make fips140 setting immutable

internal/godebug,crypto/fips140: make fips140 setting immutable

Updates #70123

Co-authored-by: qmuntal <quimmuntal@gmail.com>
Change-Id: I6a6a4656fd23ecd82428cccbd7c48692287fc75a
Reviewed-on: https://go-review.googlesource.com/c/go/+/657116
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>