build: make built dev-app web package writable

[devversion]

Bazel by default marks output files as readonly to ensure hermeticity.
Since we moved these files out of the bazel-bin directory, we should
make the files writable. This is necessary so that subsequent runs of this
script can delete the old distPath contents.

[josephperrott]

LGTM

[jelbourn]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: tests_browserstack" is failing
     status "ci/circleci: tests_saucelabs" is pending

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

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