fetchFromBitBucket: auto-remove an impure file

Also fix the hash in goPackages.inflect, the only user of the fetcher ATM.
Closes #12002 (different `inflect` fix), fixes #12012.
Using fetchzip-derived functions is likely more efficient than fetchhg,
and it's lighter on dependencies (hash is the same as with fetchhg in this case).

pkgs/build-support/fetchzip/default.nix

···
 { # Optionally move the contents of the unpacked tree up one level.
   stripRoot ? true
 , url
+, extraPostFetch ? ""
 , ... } @ args:
 
 lib.overrideDerivation (fetchurl ({
···
       fi
     '' else ''
       mv "$unpackDir"/* "$out/"
-    '');
-} // removeAttrs args [ "stripRoot" ]))
+    '') #*/
+    + extraPostFetch;
+} // removeAttrs args [ "stripRoot" "extraPostFetch" ]))
 # Hackety-hack: we actually need unzip hooks, too
 (x: {nativeBuildInputs = x.nativeBuildInputs++ [unzip];})

pkgs/top-level/all-packages.nix

···
     inherit name sha256;
     url = "https://bitbucket.org/${owner}/${repo}/get/${rev}.tar.gz";
     meta.homepage = "https://bitbucket.org/${owner}/${repo}/";
+    extraPostFetch = ''rm -f "$out"/.hg_archival.txt''; # impure file; see #12002
   };
 
   # cgit example, snapshot support is optional in cgit

pkgs/top-level/go-packages.nix

···
       rev    = "8961c3750a47b8c0b3e118d52513b97adf85a7e8";
       owner  = "pkg";
       repo   = "inflect";
-      sha256 = "04iln5b9iyvr6bcn5cj2zq6vqk3a2b4vsy48y2kylhk7qnakip99";
+      sha256 = "11qdyr5gdszy24ai1bh7sf0cgrb4q7g7fsd11kbpgj5hjiigxb9a";
     };
   };