diff --git a/tests/conformance/conftest.py b/tests/conformance/conftest.py
index 80bf0ea..411536e 100644
--- a/tests/conformance/conftest.py
+++ b/tests/conformance/conftest.py
@@ -21,14 +21,24 @@
 
 
 @pytest.fixture
-def plain_environment() -> dict[str, str]:
-    """The environment with any compiler launcher taken off `PATH`."""
+def plain_toolchain(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Takes any compiler launcher off `PATH` for the whole test.
+
+    Not just for the recording: CMake and Meson resolve the compiler when they
+    configure, and write the path they found into the build files, so a
+    launcher they saw would be used however the recording is run.
+    """
     directories = [
         directory
         for directory in os.environ.get("PATH", "").split(os.pathsep)
         if not LAUNCHERS & set(Path(directory).parts)
     ]
-    return dict(os.environ, LC_ALL="C", PATH=os.pathsep.join(directories))
+    monkeypatch.setenv("PATH", os.pathsep.join(directories))
+    # Meson goes looking for a launcher binary rather than taking the one in
+    # front of the compiler on PATH, and only leaves the compiler alone when
+    # it is told which one to use.
+    monkeypatch.setenv("CC", "cc")
+    monkeypatch.setenv("CXX", "c++")
 
 
 def pytest_addoption(parser: pytest.Parser) -> None:
diff --git a/tests/conformance/test_build_systems.py b/tests/conformance/test_build_systems.py
index 7fe5d12..6d92532 100644
--- a/tests/conformance/test_build_systems.py
+++ b/tests/conformance/test_build_systems.py
@@ -200,9 +200,7 @@ def tool_of(pid: int) -> str:
 
 
 @pytest.mark.parametrize("case", sorted(CASES))
-def test_build_system(
-    case, buildprof: Path, tmp_path: Path, request, plain_environment: dict
-):
+def test_build_system(case, buildprof: Path, tmp_path: Path, request, plain_toolchain):
     for tool in REQUIRED_TOOLS[case]:
         if shutil.which(tool) is None:
             message = f"{tool} is not installed in this environment"
@@ -216,13 +214,14 @@ def test_build_system(
     command = build(project)
 
     trace = tmp_path / f"{case}.pftrace"
+    env = dict(os.environ, LC_ALL="C")
     result = subprocess.run(
         [str(buildprof), "-o", str(trace), "--", *command],
         cwd=project,
         text=True,
         capture_output=True,
         timeout=300,
-        env=plain_environment,
+        env=env,
     )
     assert result.returncode == 0, f"{case} build failed:\n{result.stdout}{result.stderr}"
     assert trace.is_file(), f"{case} produced no trace"
@@ -242,7 +241,7 @@ def test_build_system(
     )
 
 
-def test_npm_build(buildprof: Path, tmp_path: Path, plain_environment: dict):
+def test_npm_build(buildprof: Path, tmp_path: Path, plain_toolchain):
     for tool in ("npm", "node"):
         if shutil.which(tool) is None:
             message = f"{tool} is not installed in this environment"
@@ -255,7 +254,7 @@ def test_npm_build(buildprof: Path, tmp_path: Path, plain_environment: dict):
     result = subprocess.run(
         [str(buildprof), "--no-open", "-o", str(trace), "--", *command],
         cwd=tmp_path,
-        env=dict(plain_environment, npm_config_cache=str(tmp_path / ".npm")),
+        env=dict(os.environ, LC_ALL="C", npm_config_cache=str(tmp_path / ".npm")),
         text=True,
         capture_output=True,
         timeout=60,