fix(tests): always try installing pinned solc #812

Job	Run time
clippy	54s
docs	31s
feature-checks	1m 19s
doctest	1m 2s
fmt	15s
test stable (ubuntu-latest)	1m 14s
test stable --all-features (ubuntu-latest)	1m 17s
test 1.70 (ubuntu-latest)	32s
test stable (macos-latest)	1m 7s
test stable --all-features (macos-latest)	1m 20s
test 1.70 (macos-latest)	51s
test stable (windows-latest)	1m 12s
test stable --all-features (windows-latest)	1m 11s
test 1.70 (windows-latest)	1m 4s
	13m 49s

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