feat: don't rebuild dependencies(of the crate) if the hash changes

It used to rebuild all of the dependencies of the crate, because I
embed the hash in the program, and when commiting there is a new hash.
("revision" or "hash", it is just the git hash)

Now it only rebuilds the final crate, so there is caching again. :)

flake.nix

@@ -41,12 +41,6 @@
             # Additional darwin specific inputs can be set here
             pkgs.libiconv
           ];
-
-          # Additional environment variables can be set directly
-          # MY_CUSTOM_VAR = "some value";
-          COMMIT_HASH = builtins.substring 0 7 (
-            if self ? rev then self.rev else "NoDHash"
-          );
         };
 
         craneLibLLvmTools = craneLib.overrideToolchain
@@ -64,6 +58,12 @@
         # artifacts from above.
         my-crate = craneLib.buildPackage (commonArgs // {
           inherit cargoArtifacts;
+
+          # Additional environment variables can be set directly
+          # MY_CUSTOM_VAR = "some value";
+          COMMIT_HASH = builtins.substring 0 7 (
+            if self ? rev then self.rev else "NoDHash"
+          );
         });
       in
       {