Drop the rustup-specific calling in ci/rustfmt.sh

The +rustversion call semantics are specific to rustup and do not
work with standard rust toolchains. However, because rustfmt
formatting differs slightly between stable and our 1.63 target, we
need to keep the +1.63.0 for rustup users.

Thus, here, we check for the presence of a `rustup` command and
pass the "+1.63.0" argument if we find one.

ci/rustfmt.sh

@@ -4,10 +4,16 @@ set -eox pipefail
 # Generate initial exclusion list
 #find . -name '*.rs' -type f |sort >rustfmt_excluded_files
 
+# The +rustversion syntax only works with rustup-installed rust toolchains,
+# not with any distro-provided ones. Thus, we check for a rustup install and
+# only pass +1.63.0 if we find one.
+VERS=""
+[ "$(which rustup)" != "" ] && VERS="+1.63.0"
+
 # Run fmt
 TMP_FILE=$(mktemp)
 find . -name '*.rs' -type f |sort >$TMP_FILE
 for file in $(comm -23 $TMP_FILE rustfmt_excluded_files); do
 	echo "Checking formatting of $file"
-	rustfmt +1.63.0 --check $file
+	rustfmt $VERS --check $file
 done