Enable `always-allow-substitutes` by default #539

Job	Run time
act-support (ubuntu-latest)	1m 27s
extra-nix-config (ubuntu-latest)	22s
flakes (ubuntu-latest)	39s
custom-nix-path (ubuntu-latest)	17s
extra-nix-config (macos-latest)	52s
flakes (macos-latest)	1m 4s
custom-nix-path (macos-latest)	43s
installer-options (ubuntu-latest)	20s
oldest-supported-installer (ubuntu-latest)	22s
installer-options (macos-latest)	5s
simple-build (ubuntu-latest)	22s
oldest-supported-installer (macos-latest)	52s
simple-build (macos-latest)	1m 8s
	8m 33s

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