ci: bump Emacs to 29.4 #1489

Job	Run time
test (ubuntu-latest, 27.2, false)	2m 26s
test (ubuntu-latest, 28.2, false)	4m 16s
test (ubuntu-latest, 29.4, false)	4m 6s
test (macos-latest, 27.2, false)	2m 23s
test (macos-latest, 28.2, false)	4m 3s
test (macos-latest, 29.4, false)	3m 55s
test (windows-latest, 27.2, false)	2m 30s
test (windows-latest, 28.2, false)	4m 19s
test (windows-latest, 29.4, false)	4m 4s
test (ubuntu-latest, snapshot, true)	2m 46s
test (macos-latest, snapshot, true)	3m 7s
test (windows-latest, snapshot, true)	3m 14s
	41m 9s

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