cd: add CentOS 8

Similarly to commit 8485060 ("cd: add Ubuntu Noble") it is
sometimes convenient to have the `tt` package right in the distro
specific repository even if it is available in the general
`linux-rpm/static` repository.

Since it costs nothing (the same RPM file is published to different
repositories), I propose to publish the `tt` package to the `el/8`
repository as well.

See tarantool/installer.sh#42 for the story behind it.

Fixes #1192

Author: Totktonada

magefile.publish.go

@@ -20,6 +20,7 @@ type Distro struct {
 
 var targetDistros = []Distro{
 	{OS: "el", Dist: "7"},
+	{OS: "el", Dist: "8"},
 
 	{OS: "fedora", Dist: "34"},
 	{OS: "fedora", Dist: "35"},