${platformio.packages_dir} resolves to a backslash path on Windows.
PlatformIO parses build_flags with POSIX shell rules, which strip the
backslashes and turn the NimBLE include paths into broken relative
paths (d:platformiopackages/...), failing the build with
'host/ble_gap.h: No such file or directory'. Quoting the flags
preserves backslashes through parsing.