convert/gem/ruby: Don't overwrite existing melange files #2973

Job	Run time
Build melange and add to artifact cache	33s
Build packages (bubblewrap, hello-wolfi)	1m 42s
Build packages (bubblewrap, glibc)	13m 38s
Build packages (bubblewrap, tini)	1m 10s
Build packages (bubblewrap, lzo)	1m 24s
Build packages (bubblewrap, bubblewrap)	1m 13s
Build packages (bubblewrap, gitsign)	2m 37s
Build packages (bubblewrap, guac)	3m 55s
Build packages (bubblewrap, mdbook)	3m 55s
Build packages (bubblewrap, s3cmd)	1m 10s
Build packages (bubblewrap, perl-yaml-syck)	1m 16s
Build packages (bubblewrap, ncurses)	2m 15s
Build packages (qemu, hello-wolfi)	2m 6s
Build packages (qemu, glibc)	13m 48s
Build packages (qemu, tini)	1m 43s
Build packages (qemu, lzo)	2m 21s
Build packages (qemu, bubblewrap)	1m 52s
Build packages (qemu, gitsign)	3m 48s
Build packages (qemu, guac)	5m 7s
Build packages (qemu, mdbook)	4m 42s
Build packages (qemu, s3cmd)	1m 40s
Build packages (qemu, perl-yaml-syck)	2m 7s
Build packages (qemu, ncurses)	2m 38s
	1h 16m 40s

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