CI: Pin GMT to a specific build on RTD

[seisman]

Description of proposed changes

The ReadTheDocs build started to fail recently. For example, in #2678, build https://readthedocs.org/projects/pygmt-dev/builds/21875786/ succeed but the next build https://readthedocs.org/projects/pygmt-dev/builds/21881327/ failed with the following errors:

gmt: error while loading shared libraries: libabsl_cord.so.2301.0.0: cannot open shared object file: No such file or directory

Comparing these two builds:

> + gtest 1.14.0 h00ab1b0_1 conda-forge/linux-64 395 KB85c86< + libabseil 20230125.3 cxx17_h59595ed_0 conda-forge/linux-64 1 MB---> + libabseil 20230802.0 cxx17_h59595ed_3 conda-forge/linux-64 1 MB100c101< + libgcc-ng 13.1.0 he5830b7_0 conda-forge/linux-64 758 KB---> + libgcc-ng 13.2.0 h807b86a_0 conda-forge/linux-64 753 KB102,107c103,108< + libgfortran-ng 13.1.0 h69a702a_0 conda-forge/linux-64 23 KB< + libgfortran5 13.1.0 h15d22d2_0 conda-forge/linux-64 1 MB< + libglib 2.76.4 hebfc3b9_0 conda-forge/linux-64 3 MB< + libgomp 13.1.0 he5830b7_0 conda-forge/linux-64 409 KB< + libgoogle-cloud 2.12.0 h840a212_1 conda-forge/linux-64 44 MB< + libgrpc 1.56.2 h3905398_1 conda-forge/linux-64 6 MB---> + libgfortran-ng 13.2.0 h69a702a_0 conda-forge/linux-64 23 KB> + libgfortran5 13.2.0 ha4646dd_0 conda-forge/linux-64 1 MB> + libglib 2.78.0 hebfc3b9_0 conda-forge/linux-64 3 MB> + libgomp 13.2.0 h807b86a_0 conda-forge/linux-64 412 KB> + libgoogle-cloud 2.12.0 h8d7e28b_2 conda-forge/linux-64 44 MB> + libgrpc 1.57.0 ha4d0f93_1 conda-forge/linux-64 6 MB118c119< + libprotobuf 4.23.3 hd1fb520_1 conda-forge/linux-64 2 MB---> + libprotobuf 4.23.4 hf27288f_5 conda-forge/linux-64 2 MB124c125< + libstdcxx-ng 13.1.0 hfd8a6a1_0 conda-forge/linux-64 4 MB---> + libstdcxx-ng 13.2.0 h7e041cc_0 conda-forge/linux-64 4 MB254c255< + zstd 1.5.5 hfc55251_0 conda-forge/linux-64 532 KB
\ No newline at end of file
---> + zstd 1.5.5 hfc55251_0 conda-forge/linux-64 532 K

The main difference is libabseil (20230125.3->20230802.0), which is likely due to the ongoing conda-forge migration https://conda-forge.org/status/#libabseil20230802_libgrpc157_libprotobuf4234.

For unknown reasons, the ReadTheDocs CI is using gmt-6.4.0_hf1dbac9_15 (the latest build) while the Docs workflow is using gmt-6.4.0_h4733502_10, which still works. Thus, in this PR, we can temporarily pin GMT to the specific build h4733502_10 to make the RTD CI work again.

[seisman]

OK. After pining GMT to build h4733502_10, now the RTD CI works again. Other builds between h4733502_10 (the one used by CI) and hf1dbac9_15 (the latest one) may also work, but I don't think it's worth the time to try.