Install efitools only on a branch that has the Secure Boot test

[mastacontrola]

This workflow is shared by every fogproject branch, but only the 1.6 line carries tests/secureboot-authvars.test.sh. On dev-branch the Install efitools step buys nothing at all — and it is not free.

It is the flakiest step in the job. A transient apt failure or a slow mirror wedges it for twenty minutes and takes the whole run with it. That is how fogproject#1170 — whose suite actually runs in 24 seconds — came to need a manual cancel and rerun:

1 Set up job:                      success
2 Check out the pull request:      success
3 Set up PHP 7.4:                  success
4 Install efitools:                in_progress   <- 20 minutes
5 Run the suite:                   pending

The fix

A probe step, same shape as the composer probe added in #19 and for the same reason:

- name: Does this branch have the Secure Boot test?
  id: sb_test
  run: |
    if [ -f tests/secureboot-authvars.test.sh ]; then
      echo 'present=true' >> "$GITHUB_OUTPUT"
    else
      echo 'present=false' >> "$GITHUB_OUTPUT"
      ... step summary ...
    fi

- name: Install efitools
  if: steps.sb_test.outputs.present == 'true'

Decided from the tree, not the branch name — the branch that has the test is a thing that will change, and the presence of the file is the actual precondition. The step summary says why it was skipped, so a missing efitools reads as a deliberate skip rather than as something that quietly did not happen.

What does not change

The 1.6 side is untouched: the file is there, the probe is true, the install runs exactly as before. secureboot-authvars.test.sh still skips itself when efitools is absent and run-all.sh still counts that as a pass — so this does not weaken enforcement anywhere it existed.