fix: stat of non-file in ib-apt.sh

[metaist]

If /var/cache/apt/pkgcache.bin doesn't exist, tests fail:

stat: cannot stat ‘/var/cache/apt/pkgcache.bin’: No such file or directory
../src/ib-apt.sh: line 34: 1459306405 -  : syntax error: operand expected (error token is "-  ")

[metaist]

This happens when apt-get update is interrupted during testing. I'm willing to revisit if there is another condition that triggers this error.

[metaist]

This only requires a simple check to make sure the file exists; otherwise, just run an update to create it.