ci: Adapt to workspace being HOME

The CoreOS CI shared lib sets `HOME` to the workspace:

coreos/coreos-ci-lib@a81bfab

and there's no easy way for it to detect when `HOME` is correctly set:

coreos/coreos-ci-lib@8574f04
coreos/coreos-ci-lib@853d5fd

For now, just work around this until we have a cleaner solution. (Though
it makes sense overall to uses `$HOME` anyway instead of hardcoding
`/root`).

.cci.jenkinsfile

@@ -14,7 +14,7 @@ parallel rpms: {
         # restore those through a build opt later on. Being able to stash RPMs directly is
         # super nice (and archiving later on will make it easy for anyone to download
         # binaries from PRs in the future) and meshes well with the following stages.
-        export PATH="/root/.cargo/bin:\$PATH"
+        export PATH="\$HOME/.cargo/bin:\$PATH"
         cargo install cbindgen
         cbindgen -c rust/cbindgen.toml -o rpmostree-rust.h rust