Flaky test: R workflows fail when Launchpad cannot serve the xtradeb PPA key

All three R workflows (R-CMD-check, pkgdown, citation-browser) fail intermittently in setup-r-dependencies, before any R code runs.

The package's system requirements resolve to chromium, so pak's pre_install runs add-apt-repository -y ppa:xtradeb/apps. That fetches the PPA signing key from Launchpad, and when Launchpad's API returns HTTP 500 the step dies with GPGKeyTemporarilyNotFoundError.

Because the three workflows share the step, one Launchpad hiccup fails all three at once, in about 90 seconds. The jobs look like a code failure but nothing was compiled or tested.

Seen on #163 (2026-08-25): all three failed, two passed on retry, pkgdown then failed the same way on two further retries.

Worth considering: install chromium without the PPA, ship or pin the signing key, or retry the sysreqs step. Whichever way, the goal is that a Launchpad outage stops blocking every PR that touches pkg-r/ or tests/shared/.

Inspect the shared setup-r-dependencies step used by the R-CMD-check, pkgdown, and citation-browser workflows, along with the pkg-r/ and tests/shared/ paths. Reproduce or trace the Launchpad HTTP 500 failure and compare the available approaches in the issue. Done means a transient Launchpad outage no longer blocks all three workflows, with the affected workflows passing reliably.