fix(deploy): drop yarn set version berry from setup step

The Setup Yarn2 step hard-coded `yarn set version berry` before
`yarn install --immutable`. This forces the latest 4.x release and
overrides whatever `packageManager` field the consumer has pinned.

When a consumer is on an older yarn (e.g. 4.14.x with yarn.lock
metadata `version: 9`) and `berry` resolves to 4.16.0 (which uses
metadata `version: 10`), `yarn install --immutable` then refuses to
write the metadata bump and the workflow fails:

    YN0028: │ -  version: 9
    YN0028: │ +  version: 10
    YN0028: │ The lockfile would have been modified by this install,
              which is explicitly forbidden.

Causes a self-failure too: the self-tests on this repo use the
`samplePythonProject*` fixtures which pin `yarn@4.14.1` with v9
lockfiles. The PR-test job for #55 was failing for exactly this
reason — has nothing to do with the smoketest changes in the
parent commits.

Fix: drop the `yarn set version berry` line. Corepack already
respects the `packageManager` field in package.json, so consumers
keep their pin. Consumers can use any yarn 4.x (or upgrade to
yarn 5 in future) without needing to wait for a workflow update.

Applies to both `deploy-to-aws-uv.yml` and `deploy-to-aws.yml`.

Author: chrisbc

.github/workflows/deploy-to-aws-uv.yml

@@ -137,7 +137,12 @@ jobs:
       - name: Setup Yarn2 package manager, install and list dependencies
         run: |
           corepack enable
-          yarn set version berry
+          # `yarn set version berry` was previously hard-coded here, but it
+          # forces the latest 4.x and overrides the consumer's `packageManager`
+          # pin. When the consumer is on a yarn whose lockfile metadata is
+          # older than the latest, `yarn install --immutable` then refuses to
+          # write the metadata bump and the workflow fails. Corepack already
+          # respects the `packageManager` field in package.json.
           yarn install --immutable
           yarn info
 

.github/workflows/deploy-to-aws.yml

@@ -141,7 +141,12 @@ jobs:
       - name: Setup Yarn2 package manager, install and list dependencies
         run: |
           corepack enable
-          yarn set version berry
+          # `yarn set version berry` was previously hard-coded here, but it
+          # forces the latest 4.x and overrides the consumer's `packageManager`
+          # pin. When the consumer is on a yarn whose lockfile metadata is
+          # older than the latest, `yarn install --immutable` then refuses to
+          # write the metadata bump and the workflow fails. Corepack already
+          # respects the `packageManager` field in package.json.
           yarn install --immutable
           yarn info