[eas-cli] Add eas env:set, deprecate env:create and env:update

[jonsamp]

Why

eas env:create and eas env:update force users to know up front whether a variable already exists, and the two commands use inconsistent flags (--name means the identifier in create but the rename target in update). This is janky. A single eas env:set that figures out create-vs-update is a much simpler mental model and mirrors the eas env:get / env:set verb pair (and gh secret set / gh variable set).

Closes ENG-15351.

How

• Added eas env:set, which looks the variable up by name + scope + environment and updates it if it exists, creates it if it doesn't (idempotent upsert). No --force, no "already exists" error, no overwrite prompt. It logs Created/Updated so the outcome is clear.
• Kept the existing flag model for now (--name + positional environment) so env:set is consistent with the rest of the env command family. A gh-style positional-name overhaul across get/set/delete is a deliberate follow-up.
• Rename stays out of set (it isn't a "set" operation); it remains available on the deprecated env:update.
• Deprecated eas env:create and eas env:update: both now print a deprecation notice pointing to env:set and say so in their descriptions, but keep their exact existing behavior/flags (incl. --force, --variable-name, rename) for backwards compatibility. They are intentionally not oclif aliases of set, because their flag surfaces are incompatible and a pure alias would break existing usage.

Follow-ups (not in this PR): documenting env:set on the docs site, the eventual removal of create/update after a deprecation window, and the positional-name overhaul.

Test Plan

• New EnvSet.test.ts covering create-when-absent, update-when-present without --force (project + account scope), the positional environment argument, and prompting for missing flags.
• Existing EnvCreate / EnvUpdate tests still pass with the added deprecation warning.
• yarn test (env suite) 24/24, yarn typecheck clean, yarn lint 0 errors, yarn fmt:check clean.
• Smoke-tested ./packages/eas-cli/bin/run env:set --help.

[linear-code]

ENG-15351

[codecov]

Codecov Report

❌ Patch coverage is 93.39623% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.76%. Comparing base (4b1f785) to head (aa9bdbb).

Files with missing lines	Patch %	Lines
packages/eas-cli/src/commands/env/set.ts	93.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4002      +/-   ##
==========================================
+ Coverage   59.67%   59.76%   +0.09%     
==========================================
  Files         941      942       +1     
  Lines       41319    41423     +104     
  Branches     8675     8713      +38     
==========================================
+ Hits        24654    24751      +97     
- Misses      16571    16578       +7     
  Partials       94       94              

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[github-actions]

✅ Thank you for adding the changelog entry!