fix some nits (#2034)

rework maybe_cargo_install(), renamed to cargo_install_unless, updated to take a command to attempt

ci/audit.sh

@@ -15,19 +15,16 @@ _() {
   "$@"
 }
 
-maybe_cargo_install() {
-  for cmd in "$@"; do
-    set +e
-    cargo "$cmd" --help > /dev/null 2>&1
-    declare exitcode=$?
-    set -e
-    if [[ $exitcode -eq 101 ]]; then
-      _ cargo install cargo-"$cmd"
-    fi
-  done
+cargo_install_unless() {
+  declare crate=$1
+  shift
+
+  "$@" > /dev/null 2>&1 || \
+    _ cargo install "$crate"
 }
 
-maybe_cargo_install audit tree
+cargo_install_unless cargo-audit cargo audit --version
+cargo_install_unless cargo-tree cargo tree --version
 
 _ cargo tree
 _ cargo audit