Use an ephemeral mint address if the client keypair is not available

Typically this can occur in a CI environment

(cherry picked from commit 8d700c3b94)

validator/src/bin/solana-test-validator.rs

@@ -143,14 +143,8 @@ fn main() {
     };
 
     let mint_address = pubkey_of(&matches, "mint_address").unwrap_or_else(|| {
-        read_keypair_file(&cli_config.keypair_path)
-            .unwrap_or_else(|err| {
-                eprintln!(
-                    "Error: Unable to read keypair file {}: {}",
-                    cli_config.keypair_path, err
-                );
-                exit(1);
-            })
+        read_keypair_file(dbg!(&cli_config.keypair_path))
+            .unwrap_or_else(|_| Keypair::new())
             .pubkey()
     });