Enable `always-allow-substitutes` by default #535

Job	Run time
simple-build (ubuntu-latest)	24s
simple-build (macos-latest)	57s
custom-nix-path (ubuntu-latest)	23s
act-support (ubuntu-latest)	1m 31s
custom-nix-path (macos-latest)	45s
extra-nix-config (ubuntu-latest)	19s
extra-nix-config (macos-latest)	59s
flakes (ubuntu-latest)	37s
installer-options (ubuntu-latest)	23s
oldest-supported-installer (ubuntu-latest)	22s
flakes (macos-latest)	1m 3s
installer-options (macos-latest)	6s
oldest-supported-installer (macos-latest)	51s
	8m 40s

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