Merged PR 59059: use AZL3 instead of Ubuntu for builds

use AZL3 instead of Ubuntu for builds

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#### AI description  (iteration 1)
#### PR Classification
This pull request updates the build pipeline configuration.

#### PR Summary
The changes modify the Linux build pool setup to leverage AZL3 images instead of Ubuntu by conditionally selecting different build agent images based on the architecture type.
- `eng/pipelines/common/xplat-setup.yml`: Replaced the fixed Ubuntu image demand with conditional expressions that assign `build.azurelinux.3.arm64` for ARM architectures and `build.azurelinux.3.amd64` for other architectures.
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Author: vseanreesermsft

eng/pipelines/common/xplat-setup.yml

@@ -168,7 +168,7 @@ jobs:
         # Official Build Linux Pool
         ${{ if and(or(in(parameters.osGroup, 'linux', 'freebsd', 'android', 'tizen'), eq(parameters.jobParameters.hostedOs, 'linux')), ne(variables['System.TeamProject'], 'public')) }}:
           name: $(DncEngInternalBuildPool)
-          demands: ImageOverride -equals 1es-ubuntu-2204
+          demands: ImageOverride -equals build.azurelinux.3.amd64
           os: linux
 
         # OSX Public Build Pool (we don't have on-prem OSX BuildPool).