[identity] Switch Identity unit tests to autoscaling CI runners

Summary:
Pretty much identical to D10328 but for Identity.

Context: https://linear.app/comm/issue/ENG-5770/office-mba-runner-out-of-storage#comment-d61c38e9

Test Plan:
See if CI passes

{F947195}

Reviewers: ashoat, ginsu, tomek, rohan, bartek, varun

Reviewed By: varun

Differential Revision: https://phab.comm.dev/D10339

.buildkite/blob_unittests.yml

@@ -1,5 +1,5 @@
 steps:
-  - label: ':nix: Blob Unittests'
+  - label: 'Blob Unittests'
     command:
       - 'apt update && apt install -y autoconf libtool build-essential cmake git'
       - './scripts/install_protobuf.sh'

.buildkite/identity_tests.yml

@@ -1,11 +1,15 @@
-env:
-  PATH: '$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:$PATH'
 steps:
-  - label: ':nix: Identity Tests (Nix)'
+  - label: 'Identity Tests'
     command:
+      - 'apt update && apt install -y autoconf libtool build-essential cmake git'
+      - './scripts/install_protobuf.sh'
       - 'cd services/identity'
-      - 'nix develop --accept-flake-config --command cargo test'
+      - 'cargo test'
     retry:
       automatic: true
+    plugins:
+      - docker#v5.3.0:
+          image: 'rust:1.74.0'
+          propagate-environment: true
     agents:
-      - 'mac=true'
+      - 'autoscaling=true'