worker/osbuild: use os-release to determine host OS

When running an osbuild job, we read `/etc/redhat-release` to get the
host OS name to attach as metadata to the job result.

Only Fedora and RHEL ship this file, which makes the osbuild job always
fail on other distributions.

The main reason to report host OS back to the worker server is due to
Koji composes and the koji-finalize job, which pushes it to Koji. The
motivation is to have enough information to potentially re-instantiate
/ identify the original builder host OS. There are no specific
requirements on the string.

Modify the code to use `/etc/os-release` to determine the host OS. Fall
back to using `linux` as the host OS, in case reading `os-release`
fails, log the error and continue with the job. The `linux` fallback is
suggested by the `os-release` spec [1]

[1] https://www.freedesktop.org/software/systemd/man/os-release.html#ID=

Co-authored-by: Achilleas Koutsou <achilleas@koutsou.net>

internal/common/distro.go

@@ -3,9 +3,7 @@ package common
 import (
 	"bufio"
 	"errors"
-	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 	"strings"
 )
@@ -31,20 +29,6 @@ func GetHostDistroName() (string, bool, bool, error) {
 	return name, beta, isStream, nil
 }
 
-// GetRedHatRelease returns the content of /etc/redhat-release
-// without the trailing new-line.
-func GetRedHatRelease() (string, error) {
-	raw, err := ioutil.ReadFile("/etc/redhat-release")
-	if err != nil {
-		return "", fmt.Errorf("cannot read /etc/redhat-release: %v", err)
-	}
-
-	//Remove the trailing new-line.
-	redHatRelease := strings.TrimSpace(string(raw))
-
-	return redHatRelease, nil
-}
-
 func readOSRelease(r io.Reader) (map[string]string, error) {
 	osrelease := make(map[string]string)
 	scanner := bufio.NewScanner(r)