install_exercism: 'exercism upgrade' fails (~/.local/bin not on PATH during install)

[bushidocodes]

Found by re-running the full install.sh in a fresh ubuntu:24.04 container after #39-#48 landed.

Problem

install_exercism installs the exercism binary to ~/.local/bin/exercism (via make), then runs exercism upgrade:

install_exercism() {
    make ~/.local/bin/exercism
    (cd ~ && exercism upgrade)
    make ~/.local/bin/configlet
}

But ~/.local/bin is only added to PATH by init_private_paths in a new shell; it isn't on PATH within install.sh's own process. On a first install the bare exercism call fails:

./install.sh: line 202: exercism: command not found

The binary still installs fine (3.5.8), so only the self-upgrade is skipped — minor, but it's a spurious error on every fresh install.

Suggested fix

Call exercism by absolute path:

(cd ~ && ~/.local/bin/exercism upgrade)