Test more reps; stop testing RUST_MIN_STACK

The previous experiment[1][2] didn't have enough of memory-related
errors to clearly show which values of the variables have an
effect, though it *looked* like the memory-related errors in
`rustc` only happened in Ubuntu 24.04 (not 22.04) and only happened
on the stable channel (not beta). That's one reason to increase the
total number of jobs in the experiment.

Another reason is that the memory-related errors are more varied.
Not all were true memory errors involving SIGSEGV and SIGBUS
anymore. Some were, same as reported in [3]. But some others were
panics, looking like this (the index and slice vary but, in each,
the start index is much larger than the length):

    thread 'rustc' panicked at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869/compiler/rustc_serialize/src/opaque.rs:269:45:
    range start index 159846347648097871 out of range for slice of length 39963722

Since the distribution of errors across jobs might also have
related to the order and times in which jobs started, for example
if there are inadvertent differences between different hosts (the
ARM64 Linux runners are in preview, so this seems plausible, though
fairly unlikely), this now expresses the repetition with two
variables: a high-order one, listed first in the matrix, and a
low-order one, listed last in the matrix.

Besides to allow more reps with the same values of the meaningful
variables, the reason to stop testing with `RUST_MIN_STACK` is that
it didn't seem to make a difference other than to change the
message shown, which suggests setting it to an even higher value.

[1]: e71b0cf
[2]: https://github.com/EliahKagan/gitoxide/actions/runs/12903958398
[3]: GitoxideLabs#1790

Author: EliahKagan

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

@@ -6,10 +6,10 @@ jobs:
   test-fast:
     strategy:
       matrix:
+        num-high: [ 0, 1, 2, 3, 4, 5, 6, 7 ]
         os-ver: [ '22.04', '24.04' ]
         channel: [ stable, beta ]  # `gix-macros::macros momo::ux` currently fails on `nightly`.
-        increase-stack: [ false, true ]
-        number: [ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' ]
+        num-low: [ 0, 1, 2, 3, 4, 5, 6, 7 ]
 
       fail-fast: false
 
@@ -24,9 +24,9 @@ jobs:
       - uses: taiki-e/install-action@v2
         with:
           tool: nextest
-      - name: Set RUST_MIN_STACK
-        if: matrix.increase-stack
-        run: echo RUST_MIN_STACK=16777216 >>"$GITHUB_ENV"
+      # - name: Set RUST_MIN_STACK
+      #   if: matrix.increase-stack
+      #   run: echo RUST_MIN_STACK=16777216 >>"$GITHUB_ENV"
       - name: Test (nextest)
         env:
           GIX_TEST_CREATE_ARCHIVES_EVEN_ON_CI: '1'