gddo-server: make GDDO_TEE_REQUESTS_TO_PKGGODEV case insensitive

Change-Id: I0dbde3213cee1e29631a5e68fee75e396c2253e4
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/277853
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>