Append to preexisting `MSYS` env var even if ill-formed #4740

Job	Run time
lint	3m 42s
check-packetline (ubuntu-latest)	5s
cargo-deny (advisories)	27s
cargo-deny (bans licenses sources)	28s
pure-rust-build	1m 58s
test	13m 47s
test-32bit (armv7-linux-androideabi)	1m 15s
test-fast (windows-latest)	13m 52s
WebAssembly (wasm32-unknown-unknown)	1m 6s
test-fast (macos-latest)	2m 42s
WebAssembly (wasm32-wasi)	45s
test-fast (ubuntu-latest)	3m 40s
Tests pass	1s
	43m 48s

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