Add PHP binary path override

[fabpot]

Closes symfony-cli/symfony-cli#599

[upsun-dispatch]

📋 PR Summary

This PR adds a SYMFONY_CLI_PHP_BINARY_PATH environment variable that lets users force a specific absolute PHP CLI binary path, bypassing automatic version selection (resolving issue #599 where multiple installations share the same version). It refactors PHP discovery in discovery.go into a reusable inspectPHPBinary helper plus a new discoverPHPPath entry point, injects the --version runner via a phpVersionOutput field for testability, and adds a sortVersions helper that rebuilds the seen index after sorting.

Changes

Layer / File(s)	Summary
override entry point
store.go	Adds the PHPBinaryPathEnvVar constant and a highest-priority branch in BestVersionForDir that discovers and selects the overridden binary; extracts sortVersions which now rebuilds the seen map after each sort.
discovery refactor
discovery.go	Splits binary inspection into inspectPHPBinary, adds discoverPHPPath for absolute/standalone binaries, and extracts runPHPVersion behind the injectable phpVersionOutput field.
tests
store_test.go	Adds tests covering the override happy path, symlink resolution, standalone binaries, and rejection of relative / non-PHP paths.
docs
README.md	Documents the new SYMFONY_CLI_PHP_BINARY_PATH override.

[upsun-dispatch]

🟡 Warning — Concurrent override calls can fatally panic, and every call redundantly re-execs and re-sorts.

The SYMFONY_CLI_PHP_BINARY_PATH branch turns BestVersionForDir — otherwise a read-only query — into a writer: s.addVersion(selected) appends to s.versions and s.sortVersions() rebuilds the s.seen map (re-evalSymlinks-ing every entry) on every call, and discoverPHPPath re-execs php --version each time. Two side effects follow: every invocation with the override set repeats the exec and full re-sort, and if two goroutines call BestVersionForDir with the override active the unsynchronized writes to s.seen can trigger Go's fatal "concurrent map writes" panic (the non-override paths never mutate this map).

[upsun-dispatch]

🔵 Info — Override of non-standard PHP layouts drops FPM/CGI server support.

On non-Windows, companion (FPM/CGI/config) discovery is enabled only when the resolved binary's parent directory is literally named bin (filepath.Base(dir) == "bin"). An override pointing at a custom-built PHP whose CLI binary is not under a bin/ directory (e.g. /opt/php8.4/php) is reported as a CLI-only installation with empty FPMPath/CGIPath even when a php-fpm/php-cgi sits alongside it, so the override silently loses FPM/CGI capability for such layouts.