watch: get flags from execArgv #3311

Job	Run time
Build slim tarball	2m 25s
aarch64-linux: with shared libraries	1h 8m 46s
aarch64-darwin: with shared libraries	1h 15m 50s
x86_64-darwin: with shared libraries	1h 30m 53s
x86_64-linux: with shared libraries	1h 16m 17s
	5h 14m 11s

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