Fix segfault in getUnitTestData() when env var not set

The previous implementation called .value() on std::optional without
checking if it had a value. When _NIX_TEST_UNIT_DATA was not set, this
would throw std::bad_optional_access or cause a segfault in code that
used the raw getenv() result.

The new implementation checks the optional first and throws an Error
with a helpful message directing users to run tests via meson. The
example includes --gdb since this situation may arise when trying to
debug tests without knowing about meson's test infrastructure.

src/libutil-test-support/include/nix/util/tests/characterization.hh

@@ -15,7 +15,11 @@ namespace nix {
  */
 static inline std::filesystem::path getUnitTestData()
 {
-    return getEnv("_NIX_TEST_UNIT_DATA").value();
+    auto data = getEnv("_NIX_TEST_UNIT_DATA");
+    if (!data)
+        throw Error(
+            "_NIX_TEST_UNIT_DATA environment variable is not set. Recommendation: use meson, example: 'meson test -C build --gdb'");
+    return std::filesystem::path(*data);
 }
 
 /**