[build] Add escalating backoff between download retry attempts

[simonrozsival]

Summary

CI builds intermittently fail in the "install OpenJDK and accept Android SDK licenses" step when dl.google.com has a brief network hiccup, e.g. build 1486699:

error MSB3923: Failed to download file
  "https://dl.google.com/android/repository/platform-36.1_r01.zip".
  The response ended prematurely. (ResponseEnded)

build-tools/scripts/DownloadFileWithRetry.targets already wraps MSBuild's <DownloadFile> in outer retry attempts (because the ResponseEnded failure is a top-level HttpIOException that MSBuild's inner Retries does not cover). But the outer attempts fired back-to-back with no delay — the build log shows all 3 attempts for a single file blowing through in ~8 seconds (22:47:09 -> 22:47:17). The existing RetryDelayMilliseconds=5000 never applies, since the inner retry never engages for ResponseEnded. So even a ~10s outage fails the build instantly.

Change

• Add a cross-platform inline Sleep task (via RoslynCodeTaskFactory, matching existing repo usage — MSBuild has no built-in sleep).
• Insert an escalating backoff between outer attempts: 30s -> 60s -> 90s -> 120s, each gated by !Exists() so it only waits when a retry is actually needed.
• Bump outer attempts from 3 -> 5.
• Update the header comment to document the new behavior.

A transient CDN outage now gets up to ~5 minutes of spaced retries to recover instead of failing in 8 seconds.

Test

• Targets file parses; the Sleep task and $([MSBuild]::Multiply(...)) produce 30000…120000 correctly.
• Real end-to-end download succeeds on first attempt with no sleeps firing (backoff correctly no-ops on success) and the stamp file is written as before.

[Copilot]

Pull request overview

This PR improves resiliency of the build-tool download pipeline by adding an outer retry backoff to DownloadFileWithRetry.targets, addressing intermittent dl.google.com mid-stream disconnects (ResponseEnded) that aren’t covered by MSBuild’s built-in <DownloadFile Retries=...> logic.

Changes:

• Adds an inline (RoslynCodeTaskFactory) Sleep MSBuild task for cross-platform delays.
• Introduces escalating backoff sleeps between outer <DownloadFile> attempts (30s → 60s → 90s → 120s).
• Increases the number of outer attempts from 3 to 5 and updates the header comment to explain the behavior.

Show a summary per file

File	Description
build-tools/scripts/DownloadFileWithRetry.targets	Adds a cross-platform sleep task and escalating backoff between outer download retry attempts to better tolerate transient CDN/network hiccups.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1