Enable `always-allow-substitutes` by default #536

Job	Run time
extra-nix-config (ubuntu-latest)	20s
act-support (ubuntu-latest)	1m 33s
extra-nix-config (macos-latest)	52s
custom-nix-path (ubuntu-latest)	24s
custom-nix-path (macos-latest)	41s
flakes (ubuntu-latest)	34s
installer-options (ubuntu-latest)	21s
flakes (macos-latest)	1m 0s
oldest-supported-installer (ubuntu-latest)	25s
simple-build (ubuntu-latest)	24s
installer-options (macos-latest)	5s
oldest-supported-installer (macos-latest)	51s
simple-build (macos-latest)	56s
	8m 26s

Provide feedback