Specify `Accept` header for `toolcache.downloadTool` #5047

Job	Run time
Diagnostic export (macos-latest, linked)	1m 28s
Diagnostic export (macos-latest, nightly-latest)	1m 37s
Diagnostic export (windows-latest, nightly-latest)	2m 37s
Diagnostic export (windows-latest, linked)	3m 38s
Diagnostic export (ubuntu-latest, linked)	1m 1s
Diagnostic export (ubuntu-latest, nightly-latest)	59s
	11m 20s

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