oeqa: reproducible: Preserve sstate mirror for first build

oeqa: reproducible: Preserve sstate mirror for first build

Preserves the SSTATE_MIRROR variable for the first build in the
reproducible test. This patch is intended to test the theory that using
the Yocto autobuilder sstate mirror will still be reproducible. The
autobuilder always does clean builds and never rebuilds recipes that
were built previously, thus building with the mirror but not
sharing tmpdir and sstate with previous builds should be reproducible.

There is no guarantee that all sstate caches are populated from clean
builds so this patch cannot be submitted to master, and I'm not
bothering sending it to the mailing list.

(From OE-Core rev: 6b8e0077339a89cb01aa40c1b367a4e41a638892)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>