cmd/v: `macos_v3_test.v` and `macos_v3_external_fallback_test.v` fail on the `-old-compiler` tools lanes — `@VEXE` is `v1_fallback`, not the V3 dispatcher

Describe the bug

cmd/v/macos_v3_test.v and cmd/v/macos_v3_external_fallback_test.v both drive @VEXE and assume it is the V3 dispatcher v. On the tools-* CI lanes that set VFLAGS=-old-compiler ... (tools-linux, tools-macos, tools-docker-ubuntu-musl in .github/workflows/tools_ci.yml), ./v -silent test-self cmd dispatches to v1_fallback, so the test binaries are compiled by — and @VEXE points at — v1_fallback. Both tests then fail:

cmd/v/macos_v3_test.v:85: fn test_macos_v3_explicit_build_reports_a_regular_input_error
  > assert result.output.trim_space() == 'builder error: help doesn't exist', result.output
    Left value  (len: 29): `builder error:  doesn't exist`
    Right value (len: 33): `builder error: help doesn't exist`

cmd/v/macos_v3_external_fallback_test.v:70: fn test_macos_v3_uses_external_v1_fallback_after_c_compilation_error
   > assert strict.exit_code != 0, strict.output
     Left value: `0`

Reproduce lines from the run confirm the binary: '/home/runner/work/v/v/v1_fallback' -silent '/home/runner/work/v/v/cmd/v/macos_v3_test.v', VFLAGS was: "-old-compiler -cc gcc".

CI: tools-linux (gcc), tools-linux (tcc), tools-macos (clang) on #28503.

Per test:

• test_macos_v3_explicit_build_reports_a_regular_input_error (added in 6eb1613ba2, #28490): runs @VEXE build help and expects the V3 message. The legacy builder prints the path as empty:

  $ ./v1_fallback build help
  builder error:  doesn't exist
  $ ./v build help
  builder error: help doesn't exist
  

  That empty path is itself a small legacy-builder bug (vlib/v/builder/compile.v:103 interpolates pref_.path, which is '' for build help — the legacy build command handling apparently does not carry help into pref.path), but the test cannot pass against v1_fallback regardless, since it also asserts -new-compiler build help behaviour.
• test_macos_v3_uses_external_v1_fallback_after_c_compilation_error (added in 16b3017bcd, #28417): its stub C compiler exec cc "$@" only when ${VEXE##*/} = v1_fallback and exits 1 otherwise, then expects V_MACOS_V3_NO_FALLBACK=1 to make the build fail. When @VEXE is v1_fallback there is no V3 stage to reject, cc runs, the build succeeds, and strict.exit_code == 0.

Reproduction Steps

VFLAGS=-old-compiler ./v test cmd/v/macos_v3_test.v cmd/v/macos_v3_external_fallback_test.v
# or, equivalently:
./v1_fallback test cmd/v/macos_v3_test.v cmd/v/macos_v3_external_fallback_test.v

Expected Behavior

The tests either (a) skip when os.file_name(@VEXE) is the v1_fallback binary (or when -old-compiler is in VFLAGS), or (b) resolve the dispatcher explicitly as os.join_path(os.dir(@VEXE), 'v' + exe_ext) instead of using @VEXE, so they exercise the V3 dispatch on every lane. macos_v3_external_fallback_test.v already has a guard for the absence of v1_fallback; it needs the symmetric one for being run by it.

Separately, the legacy builder's build <missing> message should include the path (builder error: help doesn't exist), matching V3.

Current Behavior

Both tests fail on every -old-compiler tools lane (and the run takes 80-150 s per test before failing).

Why it has not shown on master CI

The tools-* "Test tools" step has been skipped on master because "Check build-tools" fails first (#28501). #28503 fixes that gate and is the first run to reach these tests.

V version

V 0.5.2 76c0d94619

Environment details

Ubuntu 22.04 / macOS CI runners with VFLAGS=-old-compiler -cc <cc>; v1_fallback build help output reproduced locally on Windows 11 with the same commit.

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Start with cmd/v/macos_v3_test.v and cmd/v/macos_v3_external_fallback_test.v, then reproduce them with VFLAGS=-old-compiler and inspect .github/workflows/tools_ci.yml. Compare @VEXE with the dispatcher and review vlib/v/builder/compile.v:103 for the legacy error case. Done means the tests behave correctly on the tools-* lanes and the missing-path message matches the stated expectation.