cros_setup_toolchains: stop specifying rust-bootstrap's slot

This was put in place while we were still on Rust 1.47. We no longer
need to care specifically about slot 0 of rust-bootstrap.

BUG=chromium:1192744
TEST=SDK tryjob

Change-Id: I371b27d10b55600454961fe45c5f8108bcd33924
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2863224
Tested-by: George Burgess <gbiv@chromium.org>
Auto-Submit: George Burgess <gbiv@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index a3b4286..93dce28 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -73,7 +73,7 @@
 # build), so we have to delay their installation.
 HOST_POST_CROSS_PACKAGES = (
     'dev-lang/rust',
-    'dev-lang/rust-bootstrap:0',
+    'dev-lang/rust-bootstrap',
     'virtual/target-sdk-post-cross',
     'dev-embedded/coreboot-sdk',
     'dev-embedded/ti50-sdk',