libcap_progs: fix bash path

* in progs/capsh.c is the bash path hardcoded to '/bin/bash'.
  * this fix set's the bash path from PATH.

pkgs/os-specific/linux/libcap/progs-bash-path.patch

@@ -1,15 +0,0 @@
-this patch set the bash path from '/bin/bash' to '/var/run/current-system/sw/bin/bash'
-
-diff --git a/progs/capsh.c b/progs/capsh.c
-index 52336d7..fd98a03 100644
---- a/progs/capsh.c
-+++ b/progs/capsh.c
-@@ -556,7 +556,7 @@ int main(int argc, char *argv[], char *envp[])
- 	    }
- 	    printf("\n");
- 	} else if ((!strcmp("--", argv[i])) || (!strcmp("==", argv[i]))) {
--	    argv[i] = strdup(argv[i][0] == '-' ? "/bin/bash" : argv[0]);
-+	    argv[i] = strdup(argv[i][0] == '-' ? "/var/run/current-system/sw/bin/bash" : argv[0]);
- 	    argv[argc] = NULL;
- 	    execve(argv[i], argv+i, envp);
- 	    fprintf(stderr, "execve /bin/bash failed!\n");

pkgs/os-specific/linux/libcap/progs.nix

@@ -9,7 +9,10 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ libcap ];
 
-  patches = [ ./progs-bash-path.patch ];
+  prePatch = ''
+    BASH=$(type -tp bash)
+    substituteInPlace progs/capsh.c --replace "/bin/bash" "$BASH"
+  '';
 
   preConfigure = "cd progs";