Fix pony-lsp diagnostic test failure on OpenBSD (#5085)

On OpenBSD, get_compiler_exe_path relies entirely on argv0 to find the
executable (no /proc/self/exe or KERN_PROC_PATHNAME). The diagnostic
test was passing an empty string for argv0, so the compiler couldn't
locate the builtin package. Other platforms ignored argv0 and used
OS-specific mechanisms, masking the bug.

Author: SeanTAllen

tools/pony-lsp/test/_diagnostics_tests.pony

@@ -19,7 +19,7 @@ class \nodoc\ iso _DiagnosticTest is UnitTest
     let workspace_dir = Path.join(Path.dir(__loc.file()), "error_workspace")
     let harness = TestHarness.create(
       h,
-      PonyCompiler(""),
+      PonyCompiler("", try h.env.args(0)? else "" end),
       object iso is MessageHandler
         var received_diagnostics: USize = 0
         let expected_messages: Array[String] val = [