ci: configure to use rust-toolchain.toml

Signed-off-by: Dan Cross <cross@oxidecomputer.com>

Author: dancrossnyc

.github/buildomat/jobs/build-and-test-and-lint.sh

@@ -3,7 +3,7 @@
 #: name = "build-and-test"
 #: variety = "basic"
 #: target = "helios"
-#: rust_toolchain = "nightly-2021-09-01"
+#: rust_toolchain = "stable"
 #: output_rules = [
 #:	"/work/bins/*",
 #: ]