fix(osx): use console owner instead of $USER in postinstall script

[Copilot]

When Homebrew invokes the macOS .pkg installer, it explicitly clears USER, LOGNAME, and USERNAME from the environment. The postinstall script was using ${USER} to run git-credential-manager configure as the real user — but with USER unset, this ran as root, causing root to take ownership of ~/.gitconfig (or its symlink target). Subsequent installs then fail because the configure step can't write to the root-owned file.

Changes

• src/osx/Installer.Mac/scripts/postinstall: Replace ${USER} with $(stat -f%Su /dev/console) to determine the logged-in console user. Unlike environment variables, /dev/console ownership reflects the active GUI session regardless of how the process environment was sanitized.

# Before
sudo -u ${USER} "$INSTALL_DESTINATION/git-credential-manager" configure

# After
REAL_USER=$(stat -f%Su /dev/console)
sudo -u "${REAL_USER}" "$INSTALL_DESTINATION/git-credential-manager" configure

This mirrors the existing pattern in uninstall.sh (which uses logname), but logname on macOS also reads from LOGNAME — also cleared by Homebrew — making stat /dev/console the more robust choice for the installer context.

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