ci: Archive logs from /var/log too

When `crossdev` fails, we need to read the logs it put in `/var/log`.

ci/Jenkinsfile

@@ -63,6 +63,7 @@ pipeline {
             dir('tmp/portage') {
                 archiveArtifacts '*/*/temp/*.log'
             }
+            archiveArtifacts 'log/**/*'
         }
     }
 }

lib/common.sh

@@ -6,10 +6,11 @@ buildah_run_script() {
     shift
     script=$(readlink -f "$1")
     shift
-    mkdir -p binpkgs tmp
+    mkdir -p binpkgs log tmp
     buildah run \
         --mount type=cache,target=/var/db/repos/gentoo \
         --mount type=cache,target=/var/cache \
+        --mount type=bind,src="${PWD}"/log,target=/var/log,rw,z \
         --mount type=bind,src="${PWD}"/tmp,target=/var/tmp,rw,z \
         --mount type=bind,src="${PWD}"/binpkgs,target=/var/cache/binpkgs,rw,z \
         --mount type=bind,src="${script}",target=/run/script,ro,z \