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
```