We had twice in the past days questions about devel/upload being
"broken", since bash re-reads shell scripts during their run and this
file can get regenerated due to Makefile changes.

Since we only need this to be dynamically built for 3 variables, let's
make the file static and read those three variables when it is run,
instead of when it is built, which allows us to re-read the "latest
version" of these vars as well.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>