gpg_sign/selftest: Fix secmem parameter handling

gpg_sign/selftest: Fix secmem parameter handling

We keep seeing "cannot allocate memory" errors from rpm when signing packages
on the autobuilder. The following were tried:

* checking locked memory use (isn't hitting limits)
* Restricting RPM_GPG_SIGN_CHUNK to 1
* Limiting to 10 parallel do_package_write_rpm tasks
* Allowing unlimied memory overcommit
* Disabling rpm parallel compression

and the test still failed. Further invetigation showed that the --auto-expand-secmem
wasn't being passed to gpg-agent which meant the secmem couldn't be expanded hence the
errors when there was pressure on the agent.

The reason this happens is that some of the early gpg commands can start the agent
without the option and it sticks around in memory so a version with the correct
option may or may not get started.

We therefore add the option to all the key gpg calls.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>