Support env in the simpleGit builder options - #1203

Fixes #1189

Problem

As reported in #1189, supplying GIT_SSH_COMMAND (or any other variable) through the env property of the simpleGit() builder options worked in v3 but is silently ignored in v4 - the Git constructor only reads baseDir, maxConcurrentProcesses and trimmed, and env is not part of the published SimpleGitOptions typings. Users migrating from v3 get no error and no typings hint that the constructor approach stopped working.

Change

• env supplied to the simpleGit() builder is now used as the initial executor environment, using the same interface as calling .env({...}) on the created instance (single-key .env(name, value) calls merge into it, an object argument replaces it entirely).
• env is added to the SimpleGitOptions typings.
• The v4 environment guard applies identically to both approaches: guarded keys (every GIT_-prefixed key plus known-vulnerable keys such as EDITOR / PAGER) supplied through the constructor reject the tasks they are used with unless named in allowEnvironment, matching the existing .env() behaviour described in the v4 release notes.

simpleGit({
   unsafe: { allowUnsafeSshCommand: true },
   allowEnvironment: ['GIT_SSH_COMMAND'],
   env: { ...process.env, GIT_SSH_COMMAND },
});

Tests

• Unit tests (mocked child process) covering: constructor env reaching the spawned process, guarded-key rejection without allowEnvironment, the GIT_SSH_COMMAND scenario from Bug: git.env('GIT_SSH_COMMAND', ...) is ignored, while constructor env works #1189, and the merge/replace interaction between constructor env and .env().
• Integration tests against a real git process proving an allow-listed constructor env variable is honoured (commit author) and that unlisted guarded keys reject the task.
• Changeset included.