ci: fix omicron-common job for PRs from forks (#5806)

.github/buildomat/jobs/omicron-common.sh

@@ -5,7 +5,6 @@
 #: target = "helios-2.0"
 #: rust_toolchain = "1.77.2"
 #: output_rules = []
-#: skip_clone = true
 
 # Verify that omicron-common builds successfully when used as a dependency
 # in an external project. It must not leak anything that requires an external
@@ -18,10 +17,9 @@ set -o xtrace
 cargo --version
 rustc --version
 
+cd /tmp
 cargo new --lib test-project
 cd test-project
-cargo add omicron-common \
-    --git https://github.com/oxidecomputer/omicron.git \
-    --rev "$GITHUB_SHA"
+cargo add omicron-common --path /work/oxidecomputer/omicron/common
 cargo check
 cargo build --release