ditro/rhel86: set volid like in rhel9.0

This is only required in RHEL9.0, but best practice is to always pin these things
down. Also increases uniformity between distros.

Simplify a bit the volid generator by making it require `rand.Rand` rather than
`io.Reader`, and hence eliminating the need for error handling.

test/data/manifests/centos_8-aarch64-edge_simplified_installer-boot.json

@@ -1315,7 +1315,7 @@
           {
             "type": "org.osbuild.mkfs.fat",
             "options": {
-              "volid": ""
+              "volid": "a178892e"
             },
             "devices": {
               "device": {

test/data/manifests/centos_8-x86_64-edge_simplified_installer-boot.json

@@ -1358,7 +1358,7 @@
           {
             "type": "org.osbuild.mkfs.fat",
             "options": {
-              "volid": ""
+              "volid": "a178892e"
             },
             "devices": {
               "device": {

test/data/manifests/rhel_86-aarch64-edge_simplified_installer-boot.json

@@ -1312,7 +1312,7 @@
           {
             "type": "org.osbuild.mkfs.fat",
             "options": {
-              "volid": ""
+              "volid": "a178892e"
             },
             "devices": {
               "device": {

test/data/manifests/rhel_86-x86_64-edge_simplified_installer-boot.json

@@ -1355,7 +1355,7 @@
           {
             "type": "org.osbuild.mkfs.fat",
             "options": {
-              "volid": ""
+              "volid": "a178892e"
             },
             "devices": {
               "device": {