feat: lock CI to use specific Bun version

There was a regression in v1.1.23:

- oven-sh/bun#13896

That was fixed in v1.1.28, but that one contained a different
regression:

- oven-sh/bun#14029

In turn, that one was fixed in v1.1.29:

-  https://bun.sh/blog/bun-v1.1.29

Let's keep CI stable/controlled by locking the version number and only
updating it after we've done local testing. We'll use the version file
as described here:

- https://github.com/oven-sh/setup-bun#using-version-file

See also discussion of other ways to "lock" Bun version here:

- oven-sh/bun#10955

.bun-version

@@ -0,0 +1 @@
+1.1.29

.github/workflows/ci.yml

@@ -22,6 +22,8 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version-file: .bun-version
       - name: bun
         run: bun install --frozen-lockfile
       - name: bun lint
@@ -32,6 +34,8 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version-file: .bun-version
       - name: bun
         run: bun install --frozen-lockfile
       - name: bun tsc
@@ -43,6 +47,8 @@ jobs:
       - uses: actions/checkout@v3
       - uses: supercharge/[email protected]
       - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version-file: .bun-version
       - name: bun
         run: bun install --frozen-lockfile
       - name: bun test