tests/e2e-update: do not rebuild binaries

[lucab]

Avoid rebuilding the project from source, as binaries have already
been built in previous steps.

[lucab]

This still fails in CI but in a different way (possibly due to copying binaries across different environments):

+ install_uefi
+ /usr/bin/bootupctl backend install --src-root=/tmp/rootfs/ostree/deploy/fedora-coreos/deploy/3cff9961aa0aec9c34db5cfb341f35c40be181d67c2814a89ca6c364b55ea729.0 /tmp/rootfs

/usr/bin/bootupctl: /lib64/libc.so.6: version `GLIBC_2.33' not found (required by /usr/bin/bootupctl)

[cgwalters]

This has the same problem as coreos/rpm-ostree#2806 (comment)

IOW with this PR we silently stop testing the PR code at build time. I think I'd prefer to wait for f34 cosa and leave CI broken rather than silently testing half of things.

[lucab]

Ack, feel free to take over with the proper fix for this.