Add package.srcdir substitution to README #2929

Job	Run time
Build melange and add to artifact cache	35s
Build packages (bubblewrap, hello-wolfi)	1m 26s
Build packages (bubblewrap, glibc)	13m 26s
Build packages (bubblewrap, tini)	1m 12s
Build packages (bubblewrap, lzo)	1m 34s
Build packages (bubblewrap, bubblewrap)	1m 14s
Build packages (bubblewrap, gitsign)	2m 55s
Build packages (bubblewrap, guac)	4m 17s
Build packages (bubblewrap, mdbook)	4m 6s
Build packages (bubblewrap, s3cmd)	1m 12s
Build packages (bubblewrap, perl-yaml-syck)	1m 24s
Build packages (bubblewrap, ncurses)	2m 24s
Build packages (qemu, hello-wolfi)	2m 4s
Build packages (qemu, glibc)	13m 38s
Build packages (qemu, tini)	2m 8s
Build packages (qemu, lzo)	1m 53s
Build packages (qemu, bubblewrap)	1m 44s
Build packages (qemu, gitsign)	3m 51s
Build packages (qemu, guac)	4m 44s
Build packages (qemu, mdbook)	4m 25s
Build packages (qemu, s3cmd)	1m 42s
Build packages (qemu, perl-yaml-syck)	1m 57s
Build packages (qemu, ncurses)	2m 34s
	1h 16m 25s

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