fix(nodeenv): stop -p from reinstalling node that is already there

[ekalinin]

Closes #159.

The problem

nodeenv -p reinstalls node on every run. create_environment() skips the
"environment already exists" guard whenever -p is given, because the
virtualenv directory always exists:

if os.path.exists(env_dir) and not args.python_virtualenv:
    logger.info(' * Environment already exists: %s', env_dir)
    if not args.force:
        sys.exit(2)

So the -p path has no protection at all: each call downloads the tarball
again (there is no download cache) and overwrites bin/node. Reproduced on
master with a fresh python3 -m venv /tmp/nv159:

--- RUN 1 ---
 * Install prebuilt node (26.9.0) ..... done.
 * Appending data to /tmp/nv159/bin/activate
 * Appending data to /tmp/nv159/bin/activate.fish
10.6s

--- RUN 2 ---
 * Install prebuilt node (26.9.0) ..... done.
8.4s

The other half of the issue - the repeated * Appending data to .../bin/activate - no longer reproduces: writefile() returns early on
if content in c. But set_predeactivate_hook() appends through
open(..., 'a') with no such check, so bin/predeactivate grew from 65 to
130 bytes across those two runs.

What this changes

get_installed_node_version() reads the version out of the environment's own
bin/node, and create_environment() skips install_node() when it matches
the requested version:

    elif not args.force and \
            get_installed_node_version(env_dir) == parse_version(args.node):
        logger.info(' * Node.js %s is already installed, skipping '
                    '(use --force to reinstall)', args.node)

By that point main() has resolved args.node to an exact version, so the
comparison also holds for latest, lts and ranges such as ^24. Everything
after the block - install_activate(), npm, requirements, the predeactivate
hook - keeps running, so a skipped install still refreshes the activation
scripts.

Rejecting the run outright, as the issue suggests, would have broken two
supported workflows: changing the version in place (nodeenv -p --node=24)
and the nodeenv -p --prebuilt then nodeenv -p --node=system sequence that
install_activate() documents in a comment. Comparing versions keeps both
working without flags.

A shim counts as "not installed". After nodeenv -p --node=system the file at
bin/node is the SHIM script and it reports the system node's version;
without the distinction, nodeenv -p --node=26.8.1 would be skipped whenever
the system node happens to match, leaving the shim in place. The shim starts
with #! and a real node binary does not.

set_predeactivate_hook() now goes through the existing
writefile(..., append=True) instead of carrying its own open(..., 'a'),
which gets the deduplication for free.

Behaviour

Command, run twice in the same virtualenv	Second run
nodeenv -p --prebuilt	skips, logs that the version is already installed
nodeenv -p --node=24.0.0 over 26.9.0	installs 24.0.0
nodeenv -p --node=system	unchanged
nodeenv -p --prefer-system with system node present	unchanged
nodeenv -p --prebuilt --force	installs
nodeenv -p --prebuilt over a shim from --node=system	installs

Verified

Every row of that table was run against a real virtualenv. The second run
drops from 5.0s to 0.49s with no download:

=== RUN 2 (same version) ===
 * Node.js 26.9.0 is already installed, skipping (use --force to reinstall)
0.49s

bin/predeactivate stayed 65 bytes across all runs. Requesting the system
version (26.8.1) while a shim was in place installed a real Mach-O binary
rather than skipping.

Seven new unit tests cover the helper, the three install/skip branches and
hook idempotency. Full suite: 274 passed, plus both integration tests; flake8
clean.