build.plat: fix error in invoke_tool when running vivado.bat on Windows #3627

Job	Run time
test (3.9)	2m 18s
check-links	51s
document	51s
test (3.10)	2m 14s
smoke (amaranth-lang/amaranth-boards)	40s
test (3.11)	2m 41s
smoke (amaranth-lang/amaranth-stdio)	54s
test (3.12)	3m 5s
smoke (amaranth-lang/amaranth-soc)	49s
test (3.13)	2m 47s
test (pypy-3.9)	4m 53s
test (pypy-3.10)	4m 40s
test (3.14-dev)	1m 51s
publish-docs	13s
publish-docs-dev	0s
required	1s
publish-schemas	13s
publish-package	29s
publish-release	0s
	29m 30s

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