Support sh fallback for install script on Alpine

[fengmk2]

Context

The install script at https://viteplus.dev/install.sh uses bash-specific syntax (e.g., parentheses in expressions) that is incompatible with POSIX sh (Alpine's default shell).

When attempting to fall back from bash to sh in Alpine containers, the install fails with:

sh: syntax error: unexpected "(" (expecting "}")

See CI failure: https://github.com/voidzero-dev/setup-vp/actions/runs/23498007873/job/68384609190?pr=22

Current workaround

Alpine users must install bash as a prerequisite:

- run: apk add --no-cache bash curl gcompat libstdc++

Desired behavior

The install script should be POSIX-compatible so it can run with sh, removing the need to install bash in Alpine containers.