[roll] Roll fuchsia [devshell] Fix target flag forwarding in fx serve and vars.sh

When fx serve is invoked with -t <target> or when FUCHSIA_NODENAME
is set in the environment, fx serve previously built ffx_flags
without passing --target "${FUCHSIA_NODENAME}". Because ffx does not
inspect FUCHSIA_NODENAME directly, ffx repository server start fell
back to target.default.

Additionally, when fx-config-read was invoked in subshells,
fx-export-default-target in vars.sh re-read ${FUCHSIA_BUILD_DIR}.device
if FUCHSIA_NODENAME_IS_FROM_FILE was set, overriding explicit target
flags or dynamic environment settings.

1. In tools/devshell/serve: Forward --target "${FUCHSIA_NODENAME}" to
   ffx when FUCHSIA_NODENAME is non-empty and --no-device is not set.
2. In scripts/fx: Unset FUCHSIA_NODENAME_IS_FROM_FILE when -t is passed.
3. In tools/devshell/lib/vars.sh: Do not export default target from file
   if FUCHSIA_NODENAME_SET_BY_FX_FLAG is set.
4. In tools/devshell/BUILD.gn: Register fx_serve_test.
5. In tools/devshell/tests/subcommands/fx_serve_test: Add comprehensive
   tests for target flag forwarding, environment variables, --no-device
   handling, and .device file overrides.

Original-Original-Bug: 549976700
Test: fx test //tools/devshell:fx_script_tests
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1776013
Original-Original-Revision: 74b424eedc0c380f242defd22cc9c11db31c1e78
GitOrigin-RevId: 3621439f51cf070b18a1c14c5af95abde664c1d0
Change-Id: I69cc7dcc20a9230517ab48f33b8d7076058e3322
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1776993
Cr-Commit-Position: refs/heads/main@{#200053}