Be more strict about values that skip the GCE credential checks.

In commit a252fe83, I added support for a
`$NO_GCE_CHECK` environment variable whose presence would skip attempting to
use GCE credentials. Part of the motivation was symmetry with other Google
auth libraries, such as the Java client and Python's oauth2client.

However, both of those libraries are more precise about the values: they only
skip the GCE check if `$NO_GCE_CHECK` is set to `true` (case insensitive in
Java, and specifically `True` in Python).

This change just refines the checking to do a case-insensitive check for
`true`, bringing its behavior into line with the existing implementations. (I
also updated the test.)

PiperOrigin-RevId: 232965965