fossil: move install flags to the dedicated attr, manual and completion - to postInstall

also that `mkdir` was redundant, `make install` already does it

pkgs/applications/version-management/fossil/default.nix

@@ -55,10 +55,9 @@ stdenv.mkDerivation (finalAttrs: {
     export USER=nonexistent-but-specified-user
   '';
 
-  installPhase = ''
-    mkdir -p $out/bin
-    INSTALLDIR=$out/bin make install
+  installFlags = [ "INSTALLDIR=$(out)/bin" ];
 
+  postInstall = ''
     installManPage fossil.1
     installShellCompletion --name fossil.bash tools/fossil-autocomplete.bash
   '';