.github: bump macos-12 to macos-13

[ee7]

This is the newer x86_64 macOS image, and is not in beta. See here.

It would be possible to use macos-14 somewhere, building a macOS arm64 configlet asset natively rather than cross-compiling via Zig. But let's not consider that in this PR.

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To-do:

• Resolve error in the build workflow, in particular the cross-compile job for aarch64-macos-none

CC: ../nimdir/lib/std/sysrand.nim
/Users/runner/.cache/nim/configlet_r/@m..@snimdir@slib@sstd@ssysrand.nim.c:9:10: fatal error: 'Security/SecRandom.h' file not found
nimble.nim(304)          buildFromDir
    9 | #include <Security/SecRandom.h>

      |          ^~~~~~~~~~~~~~~~~~~~~~
    Error:  Build failed for the package: configlet
1 error generated.
     Info:  Nimble data file "/Users/runner/.nimble/nimbledata2.json" has been saved.
Error: execution of an external compiler program 'zigcc -c -w -ferror-limit=3 -pthread -target aarch64-macos-none -Os   -I/Users/runner/work/exercism-configlet/exercism-configlet/nimdir/lib -I/Users/runner/work/exercism-configlet/exercism-configlet/src -o /Users/runner/.cache/nim/configlet_r/@m..@snimdir@slib@sstd@ssysrand.nim.c.o /Users/runner/.cache/nim/configlet_r/@m..@snimdir@slib@sstd@ssysrand.nim.c' failed with exit code: 1

I think it just needs to pass to the linker:

-framework Security