ci: repair the integration workflow

[somethingwithproof]

Four defects in the integration workflow, found while making the same repair across plugins.

MySQL never authenticated. MYSQL_AUTH_USR: '--defaults-file=~/.my.cnf' set
in env: keeps the tilde literal, because tilde expansion happens before variable
expansion and a tilde arriving from a variable is not expanded again. Verified in a
Linux container:

from env:  [--defaults-file=~/.my.cnf]
arg seen:  [--defaults-file=~/.my.cnf]
inline:    [--defaults-file=/home/cacti/.my.cnf]

Every mysql call in the step was looking for a directory named ~. Assigning the
variable inside run: with $HOME expands it.

shellcheck SC2227. find . -exec php -l {} 2>&1 \; binds the redirect to
find rather than to php, which actionlint rejects. It is not a correctness bug:
php -l writes parse errors to stdout, so the check does catch a broken file either
way. Moving the redirect to the end clears the lint.

The Cacti checkout floated. repository: Cacti/cacti with no ref: takes
the default branch, so the plugin was tested against whatever core happened to be
that day. Pinned to release/1.2.31.

The Apache PHP module was version-suffixed. libapache2-mod-php${{ matrix.php }}
names a package the Ubuntu archive does not carry for every matrix entry. The
unversioned libapache2-mod-php pulls whichever module matches the runner. Note
#327 removes the package outright instead; either resolves the install step, and
the workflow configures Apache but never issues an HTTP request, so both work.

Variables are quoted at the point of use and actionlint passes.

No plugin code is touched.

[Copilot]

Pull request overview

This PR repairs the plugin’s GitHub Actions integration workflow to make it deterministic and fix several workflow/runtime issues encountered across plugins, without changing any plugin PHP code.

Changes:

• Pin the Cacti core checkout to release/1.2.31 to avoid testing against a moving default branch.
• Fix find -exec php -l redirection placement so actionlint/shellcheck no longer flags SC2227.
• Fix MySQL authentication by constructing --defaults-file using $HOME inside the run: block (avoids literal ~), and remove the PHP-version-suffixed Apache module install in favor of libapache2-mod-php.

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