Do the real rustc SIGSEGV test

This makes two changes, with the intent of producing a usable test:

- Removes `nightly`, since a test is currently failing on it. It
  can be tested later in case it fixes the SIGSEGV bug, if other
  changes don't help.

- Have `number` take on 16 values instead of just one. This is to
  make it possible to figure something out about how often the
  failure happens with the other variables and whether the other
  variables make a difference. This is needed because the failures
  are nondeterministic, may not even usually happen, or may happen
  less often but still happen for some combination of the other
  variables.

(See #1790 for context.)

Author: EliahKagan

.github/workflows/arm-segv-experiment.yml

@@ -7,9 +7,9 @@ jobs:
     strategy:
       matrix:
         os-ver: [ '22.04', '24.04' ]
-        channel: [ stable, beta, nightly ]
+        channel: [ stable, beta ]  # `gix-macros::macros momo::ux` currently fails on `nightly`.
         increase-stack: [ false, true ]
-        number: [ 0 ]  # FIXME: Increase, so results are meaningful.
+        number: [ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' ]
 
       fail-fast: false