Hardcode the use of the "default" profile for rustup (#525)

At some point rustup changed the default profile from "default" to
"minimal" if no other profile was specified.

This caused our pre-commit checks to fail which rely on clippy which is
not in the "minimal" profile.
We now specify our dependency on the "default" profile explicitly to be
more independent of future rustup changes.

Author: lfrancke

template/rust-toolchain.toml.j2

@@ -1,3 +1,4 @@
 # DO NOT EDIT, this file is generated by operator-templating
 [toolchain]
 channel = "{[ rust_version }]"
+profile = "default"