ci: bootstrap: define shell for buildkite user

[nektro]

progress towards #918

this should fix the musl failure in test/js/node/child_process/child_process.test.ts
as it's being created with machine.mjs/bootstrap.sh

[robobun]

❌ @nektro, your commit 073438d has 9 failures in #6393:

test/js/third_party/grpc-js/test-pick-first.test.ts - 1 failing on 🍎 13 x64

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64-baseline

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64-baseline

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 x64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 aarch64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64-baseline

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64

test/js/web/fetch/fetch.tls.test.ts - 1 failing on 🍎 14 aarch64

test/js/web/fetch/fetch.tls.test.ts - 1 failing on 🍎 13 aarch64

[nektro]

logs confirm $USER is "buildkite-agent" but $SHELL is still undefined in CI run
unsure if there's a manual process i need to run