bib: fix missing dnf detection

The existing code was checking if dnf could be run with --version
to detect if dnf is installed at all. That is wrong, dnf can
fail when it is just run with --version, e.g.:
```
Failed to open log file: /var/log/hawkey.log
4.14.0
```
But errors like this should be surfaced to the user (which
the next call to dnf will do). So instead of running dnf
just run:
```
sh -c "command -v dnf"
```
to check if its available. This will fix the test failures
in osbuild/image-builder-cli#374

Note that the detection is covered by an existing test so
no new test needs adding.

Author: mvo5

pkg/bib/container/solver.go

@@ -35,7 +35,7 @@ func forceSymlink(symlinkPath, target string) error {
 // option).
 func (c *Container) InitDNF() error {
 	/* #nosec G204 */
-	if err := exec.Command("podman", "exec", c.id, "dnf", "--version").Run(); err != nil {
+	if err := exec.Command("podman", "exec", c.id, "sh", "-c", `command -v dnf`).Run(); err != nil {
 		return ErrNoDnf
 	}