build: use dedicated strings for conformance test keys

This uses separate strings for the keys for the conformance tests versus
just deriving them from the sliced path.

This fixed an issue for me where a few of the tests were showing up as
InvalidConformance (notably the "blocks" test), even though, oddly
enough, the key/value entries do appear to be there (e.g. if you iterated
over them). I did notice, though, that we currently sliced the key from
the file entry of the directory iterator, which is closed at the end of
conformanceSteps.

build.zig

@@ -929,7 +929,7 @@ fn conformanceSteps(
         if (index == 0) continue;
 
         // Name of the conformance app and full path to the entrypoint.
-        const name = entry.name[0..index];
+        const name = try b.allocator.dupe(u8, entry.name[0..index]);
         const path = try fs.path.join(b.allocator, &[_][]const u8{
             c_dir_path,
             entry.name,