force compiling std from source if explicitly called

This allows the standard library to be compiled even with `download-rustc` enabled.
Which means it's no longer a requirement to compile `rustc` in order to compile `std`.

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

src/bootstrap/src/core/build_steps/compile.rs

@@ -136,7 +136,8 @@ impl Step for Std {
             compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
             target: run.target,
             crates,
-            force_recompile: false,
+            // Force compiling from source as it was explicitly called with `x build library (or std/sysroot)`.
+            force_recompile: !run.builder.paths.is_empty(),
             extra_rust_args: &[],
             is_for_mir_opt_tests: false,
         });