ci: authenticate to NuGet feeds before restore

The build jobs restore packages from an authenticated Azure Artifacts
feed, which requires the Azure Artifacts credential provider to be
primed first. Without it the restore falls back to anonymous access and
fails on a private feed.

Run NuGetAuthenticate once at the start of each job that restores
(Windows, macOS, Linux, and the .NET tool packaging job) so every
subsequent dotnet restore/publish is authenticated. The release-stage
NuGet publish job is unaffected: it pushes via the 1ES service
connection rather than a restore.

Assisted-by: Claude Opus 4.8
Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

.azure-pipelines/release.yml

@@ -134,6 +134,8 @@ extends:
                     artifactName: '${{ dim.runtime }}'
               steps:
                 - checkout: self
+                - task: NuGetAuthenticate@1
+                  displayName: 'Authenticate to NuGet feeds'
                 - task: PowerShell@2
                   displayName: 'Install .NET 10 SDK'
                   inputs:
@@ -282,6 +284,8 @@ extends:
                     artifactName: '${{ dim.runtime }}'
               steps:
                 - checkout: self
+                - task: NuGetAuthenticate@1
+                  displayName: 'Authenticate to NuGet feeds'
                 - task: UseDotNet@2
                   displayName: 'Use .NET 10 SDK'
                   inputs:
@@ -437,6 +441,8 @@ extends:
                     artifactName: '${{ dim.runtime }}'
               steps:
                 - checkout: self
+                - task: NuGetAuthenticate@1
+                  displayName: 'Authenticate to NuGet feeds'
                 - task: UseDotNet@2
                   displayName: 'Use .NET 10 SDK'
                   inputs:
@@ -524,6 +530,8 @@ extends:
                   artifactName: 'dotnet-tool'
             steps:
               - checkout: self
+              - task: NuGetAuthenticate@1
+                displayName: 'Authenticate to NuGet feeds'
               - task: UseDotNet@2
                 displayName: 'Use .NET 10 SDK'
                 inputs: