rpm: fixup gpg path

When invoking `rpmsign` to sign packages, rpmsign will invoke
`/usr/bin/gpg2` which may not exist.

This is because the path to gpg is looked up during the configure phase,
and it's then placed in the `$out/lib/rpm/macros` file. Those macros are
read at runtime.

```
$ grep __gpg result/lib/rpm/macros
%__gpg			/usr/bin/gpg2
```

By providing gpg at build time, we make sure rpm will use a correct path
for gpg.

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