rpmmd: rely on dnf-json shebang instead of using PATH python

RHEL doesn't have python3 in PATH by default. Instead it has /usr/libexec/
platform-python. When building the RPM, RHEL correctly mangles dnf-json's
shebang to the platform python. This commit switches osbuild-composer
to use this shebang because it should always respect target platform's
default python binary.

Fixes #745

internal/rpmmd/repository.go

@@ -241,7 +241,7 @@ func runDNF(dnfJsonPath string, command string, arguments interface{}, result in
 		arguments,
 	}
 
-	cmd := exec.Command("python3", dnfJsonPath)
+	cmd := exec.Command(dnfJsonPath)
 
 	stdin, err := cmd.StdinPipe()
 	if err != nil {