PGF preamble is not used

I am trying to define macros in my `"pgf.preamble"` so I can later change parts of the figure labels when including my figures in a LaTeX document without having to regenerate the figures. This works well in python:

```python
import numpy as np
import matplotlib
import matplotlib.pyplot as plt

matplotlib.use("pgf")

plt.rcParams.update({
"text.usetex": True,
"pgf.preamble": "\n".join([
r"\newcommand{\cmd}{t_a}",
])
})

x = np.linspace(0, 10, 100)
y = np.sin(x)

fig, ax = plt.subplots()
ax.plot(x, y, label=r"$\sin(x)$")
ax.legend()
ax.set_ylabel(L"$\cmd$")
fig.savefig("test.pdf")
```
runs and correctly puts $t_a$ into the y-label of the output PDF. In julia on the other hand:
```julia
using PyPlot

PyPlot.matplotlib.use("pgf")
rcParams = PyPlot.PyDict(PyPlot.matplotlib."rcParams")
rcParams["pgf.preamble"] = [
raw"\newcommand{\cmd}{t_a}"
]
rcParams["text.usetex"] = true
x = 0:0.1:10
y = sin.(x)
fig, ax = PyPlot.subplots()
ax.plot(x, y, label=L"$\sin(x)$")
ax.legend()
ax.set_ylabel(L"$\cmd$")
fig.savefig("test.pdf")
```
fails in the last line with

```
ERROR: PyError ($(Expr(:escape, :(ccall(#= /home/yc20910/.julia/packages/PyCall/1gn3u/src/pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw)))))
ValueError("Error measuring '\\\\sffamily\\\\fontsize{10.000000}{12.000000}\\\\selectfont \\\\(\\\\displaystyle \\\\cmd\\\\)'\nLaTeX Output:\n! Undefined control sequence.\n ...00}\\selectfont \\(\\displaystyle \\cmd \n \\)\n<*> ....000000}\\selectfont \\(\\displaystyle \\cmd\\)}\n \\typeout{\\the\\wd0,\\the\\ht0...\nNo pages of output.\nTranscript written on texput.log.\n")
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/figure.py", line 3012, in savefig
self.canvas.print_figure(fname, **kwargs)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backend_bases.py", line 2314, in print_figure
result = print_method(
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backends/backend_pgf.py", line 873, in print_pdf
self.print_pgf(tmppath / "figure.pgf", **kwargs)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backends/backend_pgf.py", line 860, in print_pgf
self._print_pgf_to_fh(file, **kwargs)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backend_bases.py", line 1643, in wrapper
return func(*args, **kwargs)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backends/backend_pgf.py", line 845, in _print_pgf_to_fh
self.figure.draw(renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 73, in draw_wrapper
result = draw(artist, renderer, *args, **kwargs)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 50, in draw_wrapper
return draw(artist, renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/figure.py", line 2803, in draw
mimage._draw_list_compositing_images(
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/image.py", line 132, in _draw_list_compositing_images
a.draw(renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 50, in draw_wrapper
return draw(artist, renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 3082, in draw
mimage._draw_list_compositing_images(
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/image.py", line 132, in _draw_list_compositing_images
a.draw(renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 50, in draw_wrapper
return draw(artist, renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/axis.py", line 1172, in draw
self.label.draw(renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 50, in draw_wrapper
return draw(artist, renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/text.py", line 685, in draw
bbox, info, descent = self._get_layout(renderer)
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/text.py", line 317, in _get_layout
w, h, d = renderer.get_text_width_height_descent(
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backends/backend_pgf.py", line 738, in get_text_width_height_descent
w, h, d = (LatexManager._get_cached_or_new()
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backends/backend_pgf.py", line 334, in get_width_height_descent
return self._get_box_metrics(_escape_and_apply_props(text, prop))
File "/home/yc20910/.local/lib/python3.8/site-packages/matplotlib/backends/backend_pgf.py", line 347, in _get_box_metrics
raise ValueError("Error measuring {!r}\nLaTeX Output:\n{}"

Stacktrace:
[1] pyerr_check
@ ~/.julia/packages/PyCall/1gn3u/src/exception.jl:75 [inlined]
[2] pyerr_check
@ ~/.julia/packages/PyCall/1gn3u/src/exception.jl:79 [inlined]
[3] _handle_error(msg::String)
@ PyCall ~/.julia/packages/PyCall/1gn3u/src/exception.jl:96
[4] macro expansion
@ ~/.julia/packages/PyCall/1gn3u/src/exception.jl:110 [inlined]
[5] #107
@ ~/.julia/packages/PyCall/1gn3u/src/pyfncall.jl:43 [inlined]
[6] disable_sigint
@ ./c.jl:473 [inlined]
[7] __pycall!
@ ~/.julia/packages/PyCall/1gn3u/src/pyfncall.jl:42 [inlined]
[8] _pycall!(ret::PyCall.PyObject, o::PyCall.PyObject, args::Tuple{String}, nargs::Int64, kw::Ptr{Nothing})
@ PyCall ~/.julia/packages/PyCall/1gn3u/src/pyfncall.jl:29
[9] _pycall!
@ ~/.julia/packages/PyCall/1gn3u/src/pyfncall.jl:11 [inlined]
[10] (::PyCall.PyObject)(args::String)
@ PyCall ~/.julia/packages/PyCall/1gn3u/src/pyfncall.jl:86
[11] top-level scope
@ REPL[41]:1
```

Start with the Julia rcParams assignment and the PyCall boundary, then compare the value reaching Matplotlib's PGF backend with the working Python example and the backend_pgf.py traceback shown here. Reproduce the example and verify that the defined LaTeX macro is available during figure rendering and that test.pdf is generated successfully.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.