guix-install.sh: Add the build users to the 'kvm' group.

Fixes <https://bugs.gnu.org/42129>.

* etc/guix-install.sh (sys_create_build_user): If a 'kvm' group exists,
add it to the guixbuilders' lists of supplementary groups.

etc/guix-install.sh

@@ -330,15 +330,20 @@ sys_create_build_user()
         _msg "${PAS}group <guixbuild> created"
     fi
 
+    if [ $(getent group kvm) ]; then
+        _msg "${INF}group kvm exists and build users will be added to it"
+	local KVMGROUP=,kvm
+    fi
+
     for i in $(seq -w 1 10); do
         if id "guixbuilder${i}" &>/dev/null; then
             _msg "${INF}user is already in the system, reset"
-            usermod -g guixbuild -G guixbuild           \
+            usermod -g guixbuild -G guixbuild${KVMGROUP}     \
                     -d /var/empty -s "$(which nologin)" \
                     -c "Guix build user $i"             \
                     "guixbuilder${i}";
         else
-            useradd -g guixbuild -G guixbuild           \
+            useradd -g guixbuild -G guixbuild${KVMGROUP}     \
                     -d /var/empty -s "$(which nologin)" \
                     -c "Guix build user $i" --system    \
                     "guixbuilder${i}";