tests/fedora32: retry dnf
tests/fedora32: retry dnf

Fedora mirrors are not very stable recently, leading to CI failures
that usually look like this:

> sudo: make: command not found

In fact it's caused by dnf failure to read metadata from mirrors:

> Errors during downloading metadata for repository 'updates':
>    - Downloading successful, but checksum doesn't match. Calculated: <....>
> Error: Failed to download metadata for repo 'updates': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

The error went undetected due to lack of exit code check.

This commit:
 - adds `set -e -u -o pipefail` so the script will fail early;
 - adds a retry loop with a sleep around dnf invocation.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

xiaochenshen/runc,kolyshkin/runc,opencontainers/runc,opencontainers/runc,kolyshkin/runc,dqminh/runc,estesp/runc,kolyshkin/runc,rhatdan/runc,opencontainers/runc,mikebrow/runc,rhatdan/runc,dqminh/runc,dqminh/runc,rhatdan/runc,rhatdan/runc,opencontainers/runc,dqminh/runc,xiaochenshen/runc,estesp/runc,mikebrow/runc,xiaochenshen/runc,kolyshkin/runc,mikebrow/runc,estesp/runc