[demo sandbox] Add env vars to plugin config

[wild-endeavor]

What changes were proposed in this pull request?

This adds two environment variables to the sandbox config FLYTE_PLATFORM_URL and FLYTE_PLATFORM_INSECURE so that these are injected into pods launched by propeller. These env vars indicate to flytekit that the system is running internally within a sandbox, and how to reach out to the demo sandbox control plane.

How was this patch tested?

Locally by building a sandbox, and registering/running eager tasks against it.

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.10%. Comparing base (92f8abb) to head (c6eb585).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6072   +/-   ##
=======================================
  Coverage   37.10%   37.10%           
=======================================
  Files        1318     1318           
  Lines      132331   132331           
=======================================
  Hits        49100    49100           
  Misses      78959    78959           
  Partials     4272     4272           

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.10% <ø> (ø)
unittests-flytecopilot	30.99% <ø> (ø)
unittests-flytectl	62.29% <ø> (ø)
unittests-flyteidl	7.23% <ø> (ø)
unittests-flyteplugins	53.82% <ø> (ø)
unittests-flytepropeller	42.63% <ø> (ø)
unittests-flytestdlib	57.59% <ø> (ø)

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