Use frozen pixi lock to avoid CI failing after repo sync (#88)

### What change is being made

Set `frozen: true` for synced repositories to avoid failing CI after the
sync.
This is fine as we test against the locked `pixi.lock` in the source of
truth.

### Why this change is being made

Better handling of repo mirrors.

### Tested

Confirmed CI is passing with mismatch in lock.

GitOrigin-RevId: 141df84f0cff3dd128732fa538602a90bf7d2cef

Author: awoll-bdai

.github/workflows/docs.yml

@@ -40,6 +40,7 @@ jobs:
         uses: prefix-dev/setup-pixi@v0.9.4
         with:
           pixi-version: latest
+          frozen: true
           cache: true
           environments: docs
 

.github/workflows/test.yml

@@ -32,6 +32,7 @@ jobs:
         uses: prefix-dev/setup-pixi@v0.9.4
         with:
           pixi-version: latest
+          frozen: true
           cache: true
           cache-write: ${{ github.event_name == 'push' && github.ref_name == 'main' }}
           environments: core
@@ -57,6 +58,7 @@ jobs:
         uses: prefix-dev/setup-pixi@v0.9.4
         with:
           pixi-version: latest
+          frozen: true
           cache: true
           cache-write: ${{ github.event_name == 'push' && github.ref_name == 'main' }}
           environments: controller
@@ -86,6 +88,7 @@ jobs:
         uses: prefix-dev/setup-pixi@v0.9.4
         with:
           pixi-version: latest
+          frozen: true
           cache: true
           cache-write: ${{ github.event_name == 'push' && github.ref_name == 'main' }}
           environments: >-
@@ -113,6 +116,7 @@ jobs:
         uses: prefix-dev/setup-pixi@v0.9.4
         with:
           pixi-version: latest
+          frozen: true
           cache: true
           cache-write: ${{ github.event_name == 'push' && github.ref_name == 'main' }}
           environments: isaaclab