github actions: update recommended Rust to 1.78.0 #4742

Job	Run time
checks (make -f Makefile fmt-ci, 1.78.0, rustfmt)	1m 4s
checks (make -f Makefile check-typos, 1.78.0, cargo)	1m 9s
checks_with_tang (TANG_URL=tang RUST_LOG=stratisd=debug make -f Makefile test-clevis-loop, 1.78.0...	12m 27s
pylint	59s
python-based-tests	3m 33s
shell-checks	45s
stratis-cli-checks	9m 47s
stratis-min-cli-checks	2m 31s
tests-with-testing-repo (HEAD, --post-test-check=verify-filesystem-symlinks --highest-revision-nu...	28m 25s
tests-with-testing-repo (v3.6.2, --highest-revision-number=6)	29m 4s
	1h 29m 44s

Provide feedback