disable precompiled rustc for "library" and "compiler" profiles

There is an ongoing discussion about this on Zulip and for now we
want to keep these disabled.

Zulip thread: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/.60download-rustc.20.3D.20'if-unchanged'.60.20for.20.60compiler.60.20profile.3F

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/defaults/config.compiler.toml

@@ -19,6 +19,7 @@ lto = "off"
 # Forces frame pointers to be used with `-Cforce-frame-pointers`.
 # This can be helpful for profiling at a small performance cost.
 frame-pointers = true
+download-rustc = false
 
 [llvm]
 # Having this set to true disrupts compiler development workflows for people who use `llvm.download-ci-llvm = true`

src/bootstrap/defaults/config.library.toml

@@ -10,6 +10,7 @@ bench-stage = 0
 incremental = true
 # Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
 lto = "off"
+download-rustc = false
 
 [llvm]
 # Will download LLVM from CI if available on your platform.