Switch download-rustc back on by default for profile = library

This was turned off because
`x test --set rust.download-rustc=true --stage=1 library/std` has been
broken for years. The workaround is simple and it's to remove
`test-stage = 1` from the `library` profile defaults.

Author: jyn514

src/bootstrap/defaults/bootstrap.library.toml

@@ -1,8 +1,4 @@
 # These defaults are meant for contributors to the standard library and documentation.
-[build]
-bench-stage = 1
-test-stage = 1
-
 [rust]
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
@@ -12,8 +8,7 @@ lto = "off"
 #
 # If compiler-affecting directories are not modified, use precompiled rustc to speed up
 # library development by skipping compiler builds.
-# FIXME: download-rustc is currently broken: https://github.com/rust-lang/rust/issues/142505
-download-rustc = false
+download-rustc = "if-unchanged"
 
 [llvm]
 # Will download LLVM from CI if available on your platform.