azd update: leverage existing install scripts for download and verification

[rajeshkamal5050]

Problem

azd update reimplements download, extraction, and platform-specific install logic that already exists in the official install scripts (install-azd.sh, aka.ms/install-azd.ps1). This duplicates logic and creates maintenance risk if download URLs, archive formats, or verification steps change.

Proposed

• Evaluate delegating the download/install step to the existing install scripts
• For Linux/macOS: invoke install-azd.sh with version and install-folder args
• For Windows: invoke install-azd.ps1 with equivalent params
• Alternatively: refactor shared logic into a common library that both install scripts and azd update consume

Tradeoffs

• Pro: single source of truth for install mechanics, reduces azd binary complexity
• Con: adds external dependency on script availability, may need offline fallback

Context

• Raised by @danieljurek in PR [Feature] azd update command, auto-update, and channel management #6942 code review — suggests not reimplementing what install scripts already handle.
• This also resolves the Windows MSI privilege escalation concern (Wallace, PR [Feature] azd update command, auto-update, and channel management #6942) — script installs currently use MSI for updates, which escalates beyond the original install method. Delegating to install scripts would match the original install method.
• Cosmetic issues like hardcoded temp file names in the current MSI path go away if this is adopted.

Parent epic: #6721