bug#24833: Hydra ignores MIPS-specific patch for Mesa on core-updates

[Mark H Weaver]

Commit f1267c872fcaed6c53d43b3ff51abb726f7418d6 on core-updates added a
patch to Mesa on MIPS only, in order to prevent unnecessary rebuilds on
other systems.  'guix' running on a MIPS system generates a derivation
that applies the patch, but the derivation generated for MIPS on Hydra
omits the patch.

The corresponding build page on Hydra is:

  https://hydra.gnu.org/build/1569732

The derivation for that build is:

  /gnu/store/vyqmm1zdhbzxjnaq0h6k312mm7ywi1bq-mesa-12.0.1.drv

which uses a source derivation:

  /gnu/store/368ns1adxyv5zhnsn5rx40k6ghd9sr39-mesa-12.0.1.tar.xz.drv

and this derivation simply downloads the source tarball.

In contrast, corresponding derivations generated by 'guix' on a
mips64el-linux machine are:

  /gnu/store/1r8m67dn07q2k75gl7rnw09i2kp4a0rd-mesa-12.0.1.drv
  /gnu/store/imyjp7sx1rzwpc9agvcnjvk28i4qcgis-mesa-12.0.1.tar.xz.drv

and the latter derivation is a 'patch-and-repack' derivation that
applies 'mesa-wayland-egl-symbols-check-mips.patch', as expected.

     Mark