ci(spanner): use airlock for presubmit jobs

[sakthivelmanii]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .kokoro/presubmit/node14/system-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/system-test.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/trampoline_v2.sh - .kokoro files are templated and should be updated in synthtool