fix: update $PATH to use $PNPM_HOME/bin, make $PNPM_DIRECTORY relative to $DDEV_APPROOT

[stasadev]

The Issue

PNPM 11 has some changes https://pnpm.io/blog/releases/11.0

• fix: store global binaries in PNPM_HOME/bin subdirectory pnpm/pnpm#11038

https://github.com/ddev/ddev-pnpm/actions/runs/25546596230/job/75019556138

not ok 1 install from directory
# (from function `assert_success' in file /home/linuxbrew/.linuxbrew/lib/bats-assert/src/assert_success.bash, line 42,
#  from function `health_checks' in file tests/test.bats, line 48,
#  in test file tests/test.bats, line 84)
#   `health_checks' failed
#
# -- command failed --
# status : 1
# output (3 lines):
#   [ERROR] The configured global bin directory "/mnt/ddev-global-cache/pnpm/bin" is not in PATH
#   Run "pnpm setup" to update your shell configuration.
#   Failed to execute command `pnpm config get store-dir --global`: exit status 1

https://pnpm.io/blog/releases/11.0#isolated-global-virtual-store-global-installs

pnpm link has been tightened too: only relative or absolute paths are accepted, --global is removed (use pnpm add -g .), and pnpm link with no arguments is removed.

How This PR Solves The Issue

• Updates $PATH to use $PNPM_HOME/bin
• Fixes $PNPM_DIRECTORY - it should always be relative to $DDEV_APPROOT, otherwise it's going to be run in the wrong place, because the ddev pnpm command has ## HostWorkingDir: true

Manual Testing Instructions

ddev add-on get https://github.com/ddev/ddev-pnpm/tarball/refs/pull/13/head
ddev restart

Automated Testing Overview

Release/Deployment Notes