ci: the manifest rewrite must work on both systems

`sed -i` without a backup suffix is GNU-only. BSD sed reads the following word
as the suffix, so on macOS the expression was taken as the file name and the
step failed with `invalid command code m`.

The defect could only be found on macOS. It is the platform for which no local
verification is available here, and the reason the two workflows are kept
identical apart from the platform and the implementation they name: a form that
works in one and not the other is a difference that should not exist.

Author: speak-agent

.github/workflows/ci.yml

@@ -92,7 +92,11 @@ jobs:
       # program exists to demonstrate.
       - name: The portable program runs
         run: |
-          sed -i 's|^openkal = ".*"$|openkal = { path = ".spec" }|' mcpp.toml
+          # Not `sed -i`: BSD sed requires a backup suffix after -i and reads
+          # the next word as one, so the GNU form fails on macOS with
+          # `invalid command code m`. This form is the same on both.
+          sed 's|^openkal = ".*"$|openkal = { path = ".spec" }|' mcpp.toml > mcpp.toml.next
+          mv mcpp.toml.next mcpp.toml
           cat > .spec/examples/portable/mcpp.toml <<'TOML'
           [package]
           name    = "portable"