lib: add randomHash helper

Similar to `lib.fakeHash` but value is different each time. This is
useful to avoid locking when rebuilding multiple packages and/or for
multiple architectures.

With `lib.fakeHash` multiple `FOD` builds on same machine would assume
that output with `fakeHash` can be produced so only one `FOD` build at
a time would be allowed to save efforts `_if_` all those builds would
produce same output with `fakeHash`. But actual reason to use `fakeHash`
is letting the build fail and produce hash mismatch error with real
hash value, so it makes more sense to let all fake hash FODs be different

Randomness code is partially borrowed from https://github.com/figsoda/rand-nix/blob/main/default.nix

Admittedly this is a little ugly, but if only used during local rebuilds
might be good enough.

lib/default.nix

@@ -169,7 +169,7 @@ let
       mergeAttrsWithFunc mergeAttrsConcatenateValues
       mergeAttrsNoOverride mergeAttrByFunc mergeAttrsByFuncDefaults
       mergeAttrsByFuncDefaultsClean mergeAttrBy
-      fakeHash fakeSha256 fakeSha512
+      fakeHash fakeSha256 fakeSha512 randomHash
       nixType imap;
     inherit (self.versions)
       splitVersion;

lib/deprecated/misc.nix

@@ -329,6 +329,7 @@ let
   fakeHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
   fakeSha256 = "0000000000000000000000000000000000000000000000000000000000000000";
   fakeSha512 = "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000";
+  randomHash = builtins.hashString "sha256" (builtins.readFile /proc/sys/kernel/random/uuid);
 
 in
 
@@ -346,6 +347,7 @@ in
     fakeHash
     fakeSha256
     fakeSha512
+    randomHash
     foldArgs
     getValue
     ifEnable