Bazelisk + Incompatible flags pipeline: support startup flags

[laszlocsomor]

--incompatible_windows_style_arg_escaping is a startup flag, but the pipeline used it as a build flag.

See https://buildkite.com/bazel/bazelisk-plus-incompatible-flags/builds/57#job-bc144aea-5307-4f65-ab8b-89553d11cc22

@laurentlb : possibly a bug in Bazelisk's --migration flag

[laurentlb]

Bazelisk (--migrate) doesn't support startup flags.

[laszlocsomor]

Can we fix that?

[laurentlb]

Action items:

• Decide how we pass the information that a flag is a startup flag (e.g. in the bug title).
• Update Bazelisk (insertArgs in https://github.com/philwo/bazelisk/blob/master/bazelisk.go).
• Make a Bazelisk release
• Update the CI image to use the new Bazelisk

[fweikert]

I'd be happy if someone could dogfood my instructions on how to update Bazelisk: https://github.com/bazelbuild/continuous-integration/pull/574/files#diff-62d13c1279fbb93b58652272e3c24c08R54

Lack of permissions is something that could happen (and that we should fix).

[philwo]

@laurentlb Do you have a volunteer who would like to work on this? I don't think we can get to it in the next time :(

[laurentlb]

Startup flags are rare (and László's flag is now enabled by default, I think) and we don't encourage them.

Proposal: The next person to add an incompatible startup flag should be the volunteer. :)

[philwo]

LGTM :D