* ci: re-enable browserstack tests ...

- Also sets "strict mode" bash with -euo pipefail at executor level.

* ci: Makefile: more compatible SHELL var

* ci: remove some trailing whitespace

* ci: remove redundant set -eux -o pipefail

- In some cases, just replace with -x where debug output might be useful.
- In most cases, don't bother with -x, it is a secret leak risk, and
  circleci prints the command body before executing it anyway.

* try specifying yarn version

* set yarn version

* Revert "try specifying yarn version"

This reverts commit 6b4e57a304d999d72d4b07468a6b4a30880f1f4c.