[rust] Fetch Rust toolchain by default on Linux.

Our very first step towards enabling Rust code in Chromium canary is to
start to distribute the Rust toolchain. Initially, we'll do that only on
Linux.

Bug: 1292038
Change-Id: Id9a514fd0585c21a0fc0c0259b04ae9844d14f40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4287201
Reviewed-by: Ben Pastene <[email protected]>
Reviewed-by: danakj <[email protected]>
Commit-Queue: Adrian Taylor <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1111454}
NOKEYCHECK=True
GitOrigin-RevId: 8a824c2359470b649333336a96ee1ff5f74a551b

Author: adetaylor

README.md

@@ -57,8 +57,7 @@ of the entry):
         'version': 'version:2@2022-01-01',
       },
     ],
-    'dep_type': 'cipd',
-    'condition': 'checkout_rust_toolchain_deps or use_rust',
+    # ...
   },
 ```
 
@@ -262,7 +261,7 @@ solutions = [
     ...
     "custom_vars": {
       "checkout_bazel": True,
-      "use_rust": True,
+      "checkout_crubit": True,
     },
   },
 ]