The http URL now results in a redirect to an https URL.
Update the URL to the new redirect URL.
Also add the `-L` flag to curl so that future redirects don't break our presubmits.

Also verify with the provided PGP signatures instead of using a more vulnerable sha1.

Bug: none. Fixing presubmits.
Change-Id: I715c45f7083bf0f5528d82ba93d008464d0d7f88
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42093Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>