feat: add always-allow-substitutes

This adds a new configuration option to Nix, `always-allow-substitutes`,
whose effect is simple: it causes the `allowSubstitutes` attribute in
derivations to be ignored, and for substituters to always be used.

This is extremely valuable for users of Nix in CI, where usually
`nix-build-uncached` is used. There, derivations which disallow
substitutes cause headaches as the inputs for building already-cached
derivations need to be fetched to spuriously rebuild some simple text
file.

This option should be a good middle-ground, since it doesn't imply
rebuilding the world, such as the approach I took in
https://github.com/NixOS/nixpkgs/pull/221048

src/libstore/globals.hh

@@ -255,6 +255,14 @@ public:
           For the exact format and examples, see [the manual chapter on remote builds](../advanced-topics/distributed-builds.md)
         )"};
 
+    Setting<bool> alwaysAllowSubstitutes{
+        this, false, "always-allow-substitutes",
+        R"(
+          If set to `true`, Nix will ignore the `allowSubstitutes` attribute in
+          derivations and always attempt to use available substituters.
+          For more information on `allowSubstitutes`, see [the manual chapter on advanced attributes](../language/advanced-attributes.md).
+        )"};
+
     Setting<bool> buildersUseSubstitutes{
         this, false, "builders-use-substitutes",
         R"(

src/libstore/parsed-derivations.cc

@@ -122,7 +122,7 @@ bool ParsedDerivation::willBuildLocally(Store & localStore) const
 
 bool ParsedDerivation::substitutesAllowed() const
 {
-    return getBoolAttr("allowSubstitutes", true);
+    return settings.alwaysAllowSubstitutes ? true : getBoolAttr("allowSubstitutes", true);
 }
 
 bool ParsedDerivation::useUidRange() const