build.zig: annotate max_rss for building the compiler

I observed some out-of-memory conditions happening on one of the CI
servers. This should make it avoid scheduling compiler builds at the
same time as other memory-intensive operations.

build.zig

@@ -578,6 +578,7 @@ fn addCompilerStep(
         .root_source_file = .{ .path = "src/main.zig" },
         .target = target,
         .optimize = optimize,
+        .max_rss = 5_200_000_000,
     });
     exe.stack_size = stack_size;
     exe.addAnonymousModule("aro", .{