Use `getBooleanInput` to parse flags #370

Job	Run time
public-cache-no-signing-key (ubuntu-latest, true)	29s
public-cache-no-signing-key (ubuntu-latest, false)	32s
installCommand (ubuntu-latest)	28s
public-cache-no-signing-key (macos-latest, true)	1m 27s
installCommand (macos-latest)	1m 30s
private-cache	0s
nix-master (ubuntu-latest)	37s
public-cache-no-signing-key (macos-latest, false)	1m 27s
nix-master (macos-latest)	1m 26s
public-cache (ubuntu-latest, true)	14s
public-cache (ubuntu-latest, false)	17s
public-cache (macos-latest, true)	1s
public-cache (macos-latest, false)	26s
	8m 54s

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