[guix] build the the source package only once

Summary:
If the source package was produced for a previous host, don't redo it, just use the existing one

With this change, it becomes necessary to manually delete the output/ dir when running the build on a different commit or on a different version, or else the source package will not be recreated. A sanity check is added to catch the case of an existing source tarball with a different release version.

Test Plan:
Check that the build fails if an old source tarball is present in outputs:

mkdir output
touch output/bitcoin-abc-0.28.9.tar.gz
contrib/guix/guix-build

rm -Rf output/
HOSTS="arm-linux-gnueabihf x86_64-linux-gnu" contrib/guix/guix-build &> ../guix.log

$ ls output/*
output/bitcoin-abc-0.28.10.tar.gz

output/arm-linux-gnueabihf:
bitcoin-abc-0.28.10-arm-linux-gnueabihf-debug.tar.gz  bitcoin-abc-0.28.10-arm-linux-gnueabihf.tar.gz  SHA256SUMS.part

output/x86_64-linux-gnu:
bitcoin-abc-0.28.10-x86_64-linux-gnu-debug.tar.gz  bitcoin-abc-0.28.10-x86_64-linux-gnu.tar.gz  SHA256SUMS.part

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15642