google-cloud-sdk: fix searching for cloud_sql_proxy on the PATH (#114488)

'gcloud sql connect' command allows to connect to a CloudSQL instance
from a local machine. In order to do so, it starts local
'cloud_sdk_proxy' instance. google-cloud-sdk expects to find one in SDK
root (installed by 'gcloud components') or on the PATH, if SDK is not
correctly installed ('.install' directory is missing).

Since google-cloud-sdk on NixOS is properly installed 'gcloud sql
connect' never looks for 'cloud_sql_proxy' on the PATH and simply
doesn't work at all.

The patch slightly modifies the check by looking not only for
'.install' directory, but for actual 'cloud_sql_proxy' binary before
falling back to the search on the PATH.

With this patch it's now possible to use 'gcloud sql connect' on NixOS,
provided that 'cloud_sql_proxy' is available either in user or system
enviroment (available in nixpkgs).

pkgs/tools/admin/google-cloud-sdk/default.nix

@@ -45,6 +45,8 @@ in stdenv.mkDerivation rec {
     ./gcloud-path.patch
     # Disable checking for updates for the package
     ./gsutil-disable-updates.patch
+    # Try to use cloud_sql_proxy from SDK only if it actually exists, otherwise, search for one in PATH
+    ./cloud_sql_proxy_path.patch
   ];
 
   installPhase = ''