Handle large yaml files on linux in tilt config

[jsturtevant]

What this PR does / why we need it:
make tilt-up breaks on linux with message:

error: fork/exec /usr/bin/sh: argument list too long"

This is because the file is echoed onto the command line

cluster-api-provider-azure/Tiltfile

Line 339 in bceee96

return str(local("echo \"{}\" | {}".format(yaml, envsubst_cmd), quiet=True))

The amount of data in the yaml string is to much on linux (I am guessing this works on mac) but eventually someone on a mac might have run into this as the yaml grows with more features.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:
The env sub command for the flavors works because the amount for a yaml is small.

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

fix tilt-up argument list too long error on linux

[k8s-ci-robot]

Hi @jsturtevant. Thanks for your PR.

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[devigned]

Nice!
/lgtm
/approve

[k8s-ci-robot]

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[devigned]

/ok-to-test