gdb/testsuite: improve test regexp in gdb_get_worker_threads

I spotted I made a small mistake in this commit:

  commit aff250145a
  Date:   Fri Nov 24 12:04:36 2023 +0000

      gdb: generate gdb-index identically regardless of work thread count

In this commit I added a new proc in testsuite/lib/gdb.exp called
gdb_get_worker_threads.  This proc uses gdb_test_multiple with two
possible patterns.  One pattern is anchored with '^', while the other
is missing the '^' which it could use.

This commit adds the missing '^'.

gdb/testsuite/lib/gdb.exp

@@ -10034,7 +10034,7 @@ proc gdb_get_worker_threads { {testname ""} } {
 	-wrap -re "^The number of worker threads GDB can use is unlimited \\(currently ($::decimal)\\)\\." {
 	    set worker_threads $expect_out(1,string)
 	}
-	-wrap -re "The number of worker threads GDB can use is ($::decimal)\\." {
+	-wrap -re "^The number of worker threads GDB can use is ($::decimal)\\." {
 	    set worker_threads $expect_out(1,string)
 	}
     }