refactor(backend-shared): fix getEnv return type inference

[ovr]

Remove explicit Record<string, (...args: any) => any> annotation from variables and use satisfies to preserve inferred types. Change getEnv to use rest parameters so ReturnType<Vars[T]> resolves to actual types (boolean, string, number, etc.) instead of any.

[codecov]

Codecov Report

❌ Patch coverage is 38.63636% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.85%. Comparing base (1e37d1f) to head (ab68220).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/cubejs-backend-shared/src/env.ts	17.39%	19 Missing ⚠️
...s/cubejs-clickhouse-driver/src/ClickHouseDriver.ts	50.00%	3 Missing ⚠️
...ackages/cubejs-prestodb-driver/src/PrestoDriver.ts	40.00%	1 Missing and 2 partials ⚠️
...kages/cubejs-bigquery-driver/src/BigQueryDriver.ts	50.00%	0 Missing and 1 partial ⚠️
...orchestrator/src/orchestrator/QueryOrchestrator.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10598      +/-   ##
==========================================
- Coverage   57.89%   57.85%   -0.05%     
==========================================
  Files         225      225              
  Lines       17587    17608      +21     
  Branches     3614     3621       +7     
==========================================
+ Hits        10182    10187       +5     
- Misses       6861     6877      +16     
  Partials      544      544              

Flag	Coverage Δ
cube-backend	57.85% <38.63%> (-0.05%)	⬇️

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