packit: Fix replacement of placeholders in post-upstream-clone

[nforro]

I'm not exactly sure why, but while the sed expression does work in Copr, it doesn't work in Packit sandbox environment when running propose-downstream, hence the failures: https://github.com/storaged-project/libblockdev/runs/32657399598

This should hopefully fix it.

[vojtechtrefny]

I was wondering why it failed, I suspected it was because I made the release from a non-default branch, but I didn't have time to investigate it further. Interestingly it worked for the previous release two months ago: https://dashboard.packit.dev/jobs/propose-downstream/10806

[nforro]

Interestingly it worked for the previous release two months ago: https://dashboard.packit.dev/jobs/propose-downstream/10806

Right, but at that time there was still sed -E: https://github.com/storaged-project/libblockdev/blob/27881cb6e57eb0146c51d032ea72fbb3cd6bda7e/.packit.yaml#L4

[vojtechtrefny]

Interestingly it worked for the previous release two months ago: https://dashboard.packit.dev/jobs/propose-downstream/10806

Right, but at that time there was still sed -E:

https://github.com/storaged-project/libblockdev/blob/27881cb6e57eb0146c51d032ea72fbb3cd6bda7e/.packit.yaml#L4

Oh, I forgot about this change. Thanks again for the fix.