ci: Reduce use of bash -c

It is confusing to treat commands as a single string. This change is
also required to support paths and strings with spaces in them in the
future.

This requires replacing TEST_RUNNER_ENV with a global export, because it
no longer works. See:

```bash
$ export ENV="A=1" && $ENV ls
bash: A=1: command not found...
```

Or in the CI task:

+ DIR_UNIT_TEST_DATA=/ci_container_base/ci/scratch/qa-assets/unit_test_data/
+ LD_LIBRARY_PATH=/ci_container_base/depends/i686-pc-linux-gnu/lib
+ BITCOIND=bitcoin-node make -j10 check VERBOSE=1
/ci_container_base/ci/test/03_test_script.sh: line 166: BITCOIND=bitcoin-node: command not found

https://github.com/bitcoin/bitcoin/pull/28954/checks?check_run_id=19096858944
https://cirrus-ci.com/task/6718317604372480

ci/test/00_setup_env.sh

@@ -46,7 +46,6 @@ export RUN_TIDY=${RUN_TIDY:-false}
 # This is needed because some ci machines have slow CPU or disk, so sanitizers
 # might be slow or a reindex might be waiting on disk IO.
 export TEST_RUNNER_TIMEOUT_FACTOR=${TEST_RUNNER_TIMEOUT_FACTOR:-40}
-export TEST_RUNNER_ENV=${TEST_RUNNER_ENV:-}
 export RUN_FUZZ_TESTS=${RUN_FUZZ_TESTS:-false}
 
 # Randomize test order.

ci/test/00_setup_env_i686_multiprocess.sh

@@ -14,4 +14,4 @@ export DEP_OPTS="DEBUG=1 MULTIPROCESS=1"
 export GOAL="install"
 export BITCOIN_CONFIG="--enable-debug CC='clang -m32' CXX='clang++ -m32' \
 LDFLAGS='--rtlib=compiler-rt -lgcc_s' CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE'"
-export TEST_RUNNER_ENV="BITCOIND=bitcoin-node"
+export BITCOIND=bitcoin-node  # Used in functional tests

ci/test/03_test_script.sh

@@ -163,15 +163,16 @@ if [ -n "$USE_VALGRIND" ]; then
 fi
 
 if [ "$RUN_UNIT_TESTS" = "true" ]; then
-  bash -c "${TEST_RUNNER_ENV} DIR_UNIT_TEST_DATA=${DIR_UNIT_TEST_DATA} LD_LIBRARY_PATH=${DEPENDS_DIR}/${HOST}/lib make $MAKEJOBS check VERBOSE=1"
+  DIR_UNIT_TEST_DATA="${DIR_UNIT_TEST_DATA}" LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" make "${MAKEJOBS}" check VERBOSE=1
 fi
 
 if [ "$RUN_UNIT_TESTS_SEQUENTIAL" = "true" ]; then
-  bash -c "${TEST_RUNNER_ENV} DIR_UNIT_TEST_DATA=${DIR_UNIT_TEST_DATA} LD_LIBRARY_PATH=${DEPENDS_DIR}/${HOST}/lib ${BASE_OUTDIR}/bin/test_bitcoin --catch_system_errors=no -l test_suite"
+  DIR_UNIT_TEST_DATA="${DIR_UNIT_TEST_DATA}" LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" "${BASE_OUTDIR}"/bin/test_bitcoin --catch_system_errors=no -l test_suite
 fi
 
 if [ "$RUN_FUNCTIONAL_TESTS" = "true" ]; then
-  bash -c "LD_LIBRARY_PATH=${DEPENDS_DIR}/${HOST}/lib ${TEST_RUNNER_ENV} test/functional/test_runner.py --ci $MAKEJOBS --tmpdirprefix ${BASE_SCRATCH_DIR}/test_runner/ --ansi --combinedlogslen=99999999 --timeout-factor=${TEST_RUNNER_TIMEOUT_FACTOR} ${TEST_RUNNER_EXTRA} --quiet --failfast"
+  # shellcheck disable=SC2086
+  LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" test/functional/test_runner.py --ci "${MAKEJOBS}" --tmpdirprefix "${BASE_SCRATCH_DIR}"/test_runner/ --ansi --combinedlogslen=99999999 --timeout-factor="${TEST_RUNNER_TIMEOUT_FACTOR}" ${TEST_RUNNER_EXTRA} --quiet --failfast
 fi
 
 if [ "${RUN_TIDY}" = "true" ]; then
@@ -200,5 +201,6 @@ if [ "${RUN_TIDY}" = "true" ]; then
 fi
 
 if [ "$RUN_FUZZ_TESTS" = "true" ]; then
-  bash -c "LD_LIBRARY_PATH=${DEPENDS_DIR}/${HOST}/lib test/fuzz/test_runner.py ${FUZZ_TESTS_CONFIG} $MAKEJOBS -l DEBUG ${DIR_FUZZ_IN} --empty_min_time=60"
+  # shellcheck disable=SC2086
+  LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" test/fuzz/test_runner.py ${FUZZ_TESTS_CONFIG} "${MAKEJOBS}" -l DEBUG "${DIR_FUZZ_IN}" --empty_min_time=60
 fi