Most runtime tests that invoke the compiler to build a sub-test binary
do so with a special environment constructed by testEnv that strips
out environment variables that should apply to the test but not to the
build.

Fix TestGdbPython to use this test environment when invoking go build,
like other tests do.

Change-Id: Iafdf89d4765c587cbebc427a5d61cb8a7e71b326
Reviewed-on: https://go-review.googlesource.com/10455Reviewed-by: Russ Cox <rsc@golang.org>