[Do not merge] Specify `Accept` header for `toolcache.downloadTool` #5048

Job	Run time
Diagnostic export (windows-latest, nightly-latest)	2m 42s
Diagnostic export (macos-latest, linked)	1m 11s
Diagnostic export (ubuntu-latest, nightly-latest)	57s
Diagnostic export (windows-latest, linked)	3m 9s
Diagnostic export (ubuntu-latest, linked)	1m 3s
Diagnostic export (macos-latest, nightly-latest)	1m 7s
	10m 9s

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