[chore] Fix loadtest setup script (#33860)

**Description:**
This PR fixes the load test pipeline which was broken for a while.
The array of tests was not iterated over correctly in
https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/.github/workflows/scripts/setup_e2e_tests.sh
and the first iteration of the loop resulted in the whole array being
added to the matrix instead of just the first entry of the array.

I introduced a different way to read in the array of tests at the start
of the setup bash script, which works now.
[source for the solution](https://stackoverflow.com/a/10586169)

**Link to tracking Issue:** Fixes #33577

**Testing:** Tested successfully on my personal fork. This is a [link to
a load test
run](https://github.com/mowies/opentelemetry-collector-contrib/actions/runs/9761599340)
where the matrix is correctly parsed by github actions.

Signed-off-by: Moritz Wiesinger <moritz.wiesinger@dynatrace.com>

.github/workflows/scripts/setup_e2e_tests.sh

@@ -4,20 +4,21 @@
 # SPDX-License-Identifier: Apache-2.0
 
 TESTS="$(make -s -C testbed list-tests | xargs echo|sed 's/ /|/g')"
-TESTS=("${TESTS//|/ }")
+IFS='|' read -r -a TEST_ARRAY <<< "$TESTS"
+
 MATRIX="{\"include\":["
 curr=""
-for i in "${!TESTS[@]}"; do
-if (( i > 0 && i % 2 == 0 )); then
-    curr+="|${TESTS[$i]}"
-else
+for i in "${!TEST_ARRAY[@]}"; do
+  if (( i > 0 && i % 2 == 0 )); then
+    curr+="|${TEST_ARRAY[$i]}"
+  else
     if [ -n "$curr" ] && (( i>1 )); then
-    MATRIX+=",{\"test\":\"$curr\"}"
+      MATRIX+=",{\"test\":\"$curr\"}"
     elif [ -n "$curr" ]; then
-    MATRIX+="{\"test\":\"$curr\"}"
+      MATRIX+="{\"test\":\"$curr\"}"
     fi
-    curr="${TESTS[$i]}"
-fi
+    curr="${TEST_ARRAY[$i]}"
+  fi
 done
 MATRIX+=",{\"test\":\"$curr\"}]}"
 echo "loadtest_matrix=$MATRIX" >> "$GITHUB_OUTPUT"