.github: fake it, let qemu target build in $HOME

Apparently, running "make test-qeneth" from an output directory that's
not in $HOME does not work in Azure.  This patch adds an exception for
the x86_64-netconf target that ensures regression tests run just as if
nothing had changed.

Other platform//variants will be built on /mnt for the time being since
we're running out of space in the runner:

df -h =================================================================
Filesystem      Size  Used Avail Use% Mounted on
/dev/root        73G   58G   16G  80% /
tmpfs           7.9G  172K  7.9G   1% /dev/shm
tmpfs           3.2G  1.1M  3.2G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sda15      105M  6.1M   99M   6% /boot/efi
/dev/sdb1        74G  4.1G   66G   6% /mnt
tmpfs           1.6G   12K  1.6G   1% /run/user/1001
=======================================================================

This is the output of the new "Manny" investigator, it revealed that we
only have 16 GiB of space, after checkout and cache donwload, to build
and upload *all* platforms/variants.  Hence the cheat with /mnt, which
we honestly don't know will remain mounted on these runners.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/build.yml

@@ -31,8 +31,12 @@ jobs:
               target=${{ matrix.platform }}-${{ matrix.variant }}
           fi
           echo "dir=infix-$target"        >> $GITHUB_OUTPUT
-          echo "out=/mnt/x-$target"       >> $GITHUB_OUTPUT
           echo "tgz=infix-$target.tar.gz" >> $GITHUB_OUTPUT
+          if [ "$target" = x86_64 ]; then
+              echo "out=$PWD/output"      >> $GITHUB_OUTPUT
+          else
+              echo "out=/mnt/x-$target"   >> $GITHUB_OUTPUT
+          fi
       - name: Restore Cache of dl/
         uses: actions/cache@v4
         with: