node: fix invalid YAML syntax in pnpm v11 storeDir setup

[lullabyX]

With --pnpm-store-version v11, _add_pnpm_config() writes storeDir=$PWD/... to pnpm-workspace.yaml which is invalid YAML, crashes pnpm:
[ERROR] can not read a block mapping entry; a multiline key may not be an implicit key

Fix: = → : . The else branch (.npmrc + store-dir=) is correct as-is and unchanged.

Introduced in #537. Reference comment

[lullabyX]

hey @bbhtt
The node CI failures are unrelated to this PR. Looks like the failures are in the Electron integration test, which doesn't go through the pnpm path

FAILED tests/test_providers.py::test_special_electron[ProviderFactoryType.NPM-24] - FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/flatpak-builder-tools/flatpak-builder-tools/cache/.flatpak-builder/build/module/node_modules/electron/dist/version'
FAILED tests/test_providers.py::test_special_electron[ProviderFactoryType.YARN-24] - FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/flatpak-builder-tools/flatpak-builder-tools/cache/.flatpak-builder/build/module/node_modules/electron/dist/version'