Add air-gapped/GHE configurability to `install.ps1` (Windows parity)

[sergio-sisternes-epam]

Summary

PR #660 adds configurable env vars to install.sh for air-gapped and GHE environments (APM_INSTALL_DIR, GITHUB_URL, APM_REPO, VERSION). The Windows installer (install.ps1) should support the same configurability so Windows users get an equivalent experience.

Proposed solution

Add the same environment variable support to install.ps1:

Variable	Default	Description
APM_INSTALL_DIR	Platform-appropriate default	Directory for the apm binary
GITHUB_URL	https://github.com	Base URL for downloads (mirrors, GHE)
APM_REPO	microsoft/apm	GitHub repository
VERSION	(latest)	Pin a specific release (skips GitHub API)

Key behaviors to match:

• When VERSION is set, skip the GitHub API and construct the download URL directly
• Derive the API URL from GITHUB_URL for GHE compatibility
• Support version pinning via parameter: irm https://aka.ms/apm-windows | iex -Args @v1.2.3 (or equivalent PowerShell pattern)
• Auth token resolution (GITHUB_APM_PAT, GITHUB_TOKEN) should work with both API and direct download paths

Acceptance criteria

• install.ps1 supports APM_INSTALL_DIR, GITHUB_URL, APM_REPO, VERSION env vars
• Version pinning skips GitHub API
• GHE API URL derived correctly from GITHUB_URL
• Documentation updated (installation page, apm-guide skill)
• CHANGELOG entry added

Related

• feat: make install.sh configurable for air-gapped environments #660 -- Unix installer configurability (this is the Windows counterpart)