Move almost everything over to Nix

Eliminate Xonsh usage
Eliminate usage of the supports.yml files for default.nix
Point ISOs directly to MinIO URLs instead

.gitlab-ci-build.yml.in

@@ -39,10 +39,7 @@ stages:
       -var memory=4096
       ${EXCEPT}
       sources
-  before_script:
-    - nix run ".#cache-restore"
   script:
-    - ls -l cache/
     - echo $COMMAND > build.sh
     - bash build.sh || exit 1
 
@@ -53,7 +50,7 @@ qemu.amd64:
     only: "-only=qemu.amd64"
     tag: qemu
   parallel:
-    matrix: []
+    matrix: QEMU_MATRIX
 
 virtualbox-iso.amd64:
   extends: .build
@@ -62,7 +59,7 @@ virtualbox-iso.amd64:
     only: '-only="virtualbox-iso.amd64"'
     tag: vbox
   parallel:
-    matrix: []
+    matrix: VBOX_MATRIX
 
 hyperv-iso.amd64:
   extends: .build
@@ -71,7 +68,7 @@ hyperv-iso.amd64:
     only: '-only="hyperv-iso.amd64"'
     tag: hyperv
   parallel:
-    matrix: []
+    matrix: HYPERV_MATRIX
   script:
     - echo $env:COMMAND > build.ps1
     - echo 'if (-not $?) { throw "Error in build"}' >> build.ps1