Enable usage of machine set CRATES var

Repos using the `run_task` script would like to set the `CRATES` env var
by using

CRATES="$(cargo metadata --no-deps --format-version 1 | jq -j -r '.packages | map(.manifest_path | rtrimstr("/Cargo.toml") | ltrimstr("'"$PWD"'/")) | join(" ")')"

I don't know exactly why but this results in some sort of square braces
data type that seems to only work in a loop using `for crate in $CRATES`
instead of the `for crate in ${CRATES[@]}` like we currently have.

ci/run_task.sh

@@ -62,7 +62,7 @@ main() {
     if [ -e "$crates_script" ]; then
         verbose_say "Sourcing $crates_script"
         . "$crates_script"
-        for crate in "${CRATES[@]}"; do
+        for crate in $CRATES; do
             verbose_say "Found crate: $crate"
         done
     else
@@ -109,7 +109,7 @@ main() {
 
 # Build and test for each crate, done with each toolchain.
 build_and_test() {
-    for crate in "${CRATES[@]}"; do
+    for crate in $CRATES; do
         local test_vars_script="$REPO_DIR/$crate/contrib/test_vars.sh"
 
         # Clean variables and also make sure they are defined.
@@ -282,7 +282,7 @@ build_docs_with_stable_toolchain() {
 do_bench() {
     verbose_say "Running bench tests for: $CRATES"
 
-    for crate in "${CRATES[@]}"; do
+    for crate in $CRATES; do
         pushd "$REPO_DIR/$crate" > /dev/null
         # Unit tests are ignored so if there are no bench test then this will just succeed.
         RUSTFLAGS='--cfg=bench' cargo bench