Make `do_ci.sh check_format` run outside of CI (#9791)

* Make `do_ci.sh check_format` run outside of CI

With this change, this now works:

```
CLANG_FORMAT=/usr/local/Cellar/llvm/9.0.0_1/bin/clang-format \
BUILDIFIER_BIN=~/go/bin/buildifier \
BUILDOZER_BIN=~/go/bin/buildozer \
LLVM_ROOT=/usr/local/opt/llvm/ \
NUM_CPUS=2 \
BUILD_DIR=/tmp \
ENVOY_SRCDIR=$(pwd) \
bash -x ci/do_ci.sh check_format
```

which is very handy for validating PRs locally without using
docker.

Note: this will only work if nothing in your $PATH contains
spaces, which breaks `./tools/proto_format.sh check`. I'll
fix that in a follow-up.

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>