chore: GitHub Actions: move boards to experimental

The Olimex iMX8 and Jetson boards have been failing to build for some time now, hence move them to experimental.

.github/workflows/build.yml

@@ -50,25 +50,29 @@ jobs:
  vexpress-qemu-flash,
  x86-virtual,
  beaglebone-uboot,
- olimex-imx8mp-evb,
- jetson-agx-orin-devkit,
- jetson-agx-xavier-devkit,
- jetson-orin-nano-devkit
  ]
  experimental: [false]
  subpath: [.]
  include:
  - board: raspberrypi4-64-app-updates
  subpath: demos
  experimental: true
- - board: imx93-var-som
- experimental: true
  - board: qemuarm64-client-only
  subpath: demos
  experimental: false
  - board: qemuarm64-bootloader-validation
  subpath: demos
  experimental: false
+ - board: imx93-var-som
+ experimental: true
+ - board: olimex-imx8mp-evb
+ experimental: true
+ - board: jetson-agx-orin-devkit
+ experimental: true
+ - board: jetson-agx-xavier-devkit
+ experimental: true
+ - board: jetson-orin-nano-devkit
+ experimental: true
 
  runs-on: [self-hosted, linux, x64]
  container: