build.fsx: fix working dir when uploading nugets #1027

Job	Run time
buildAndTest (ubuntu-latest)	3m 38s
buildAndTest (windows-latest)	5m 40s
buildAndTest (macOS-latest)	2m 58s
deployReleaseBinaries	6m 57s
deployReleaseDocs	1m 11s
	20m 24s

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