watch: get flags from execArgv #3381

Job	Run time
Build slim tarball	2m 23s
aarch64-darwin: with shared libraries	38m 55s
aarch64-linux: with shared libraries	28m 11s
x86_64-linux: with shared libraries	34m 1s
x86_64-darwin: with shared libraries	42m 59s
	2h 26m 29s

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