dotnet: silence mime type warning, make build (more) deterministic #805

Job	Run time
Test Browser on Linux (chrome, main)	27s
Test Browser on macOS (safari, default)	1m 17s
Test Shell (v8, disabled)	1m 36s
Test Shell (spidermonkey, main)	44s
Test Shell (spidermonkey, default)	5m 15s
Test Browser on macOS (safari, disabled)	1m 6s
Test Browser on macOS (safari, main)	47s
Test Browser on Linux (chrome, disabled)	58s
Test Shell (v8, main)	39s
Test Shell (jsc, disabled)	1m 45s
Test Shell (v8, default)	2m 43s
Test Browser on Linux (chrome, default)	1m 7s
Test Browser on Linux (firefox, main)	1m 11s
Test Shell (jsc, main)	39s
Test Build	1m 48s
Test Browser on Linux (firefox, default)	1m 33s
Test Shell (jsc, default)	3m 54s
Test Shell (spidermonkey, disabled)	2m 52s
Test Browser on Linux (firefox, disabled)	1m 14s
	31m 35s

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