fix: don't leak cwd from install_exercism

[bushidocodes]

Problem

install_exercism changed the working directory and never restored it:

install_exercism() {
    cd ~ || exit            # leaks $HOME as cwd for everything after
    make ~/.local/bin/exercism
    exercism upgrade
    make ~/.local/bin/configlet
}

Every later step that runs make from the repo root then breaks. In master's main() order the next make-based step, install_wasmtime, failed:

make: *** No rule to make target '/root/.wasmtime/bin/wasmtime'.  Stop.

(Confirmed it's cwd, not a Makefile bug — the rule matches fine when make runs from the repo directory.)

Fix

Run both make targets from the repo dir, and scope the cd ~ to a subshell for exercism upgrade:

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

Found by running the full install.sh in a fresh ubuntu:24.04 container (#41).

Fixes #41

🤖 Generated with Claude Code