[intel-mkl] Update description field

[NancyLi1013]

Describe the pull request

• What does your PR fix?

  Fixes [intel-mkl] build failure using GitHub Actions (MSBuild based project) #18896

Add the information that users need to install MKL manually to description field.

Note that please download and install MKL from https://registrationcenter.intel.com/en/products/download/3178/ manually before using it in vcpkg.

Note: No feature needs to test.

[Neumann-A]

Probably:
https://software.intel.com/content/www/us/en/develop/tools/oneapi/base-toolkit/download.html?operatingsystem=window&distributions=webdownload&options=offline

[github-actions]

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After committing all other changes, the version database must be updated

git add -u && git commit
git checkout 57bd7102d9fd880daa1b0958692294c4a125f6d8 -- versions
./vcpkg x-add-version --all

Diff

diff --git a/versions/i-/intel-mkl.json b/versions/i-/intel-mkl.json
index 36f844b..1c5ffb0 100644
--- a/versions/i-/intel-mkl.json
+++ b/versions/i-/intel-mkl.json
@@ -1,7 +1,7 @@
 {
   "versions": [
     {
-      "git-tree": "88fbc68eed77db424c50b2d4796a6478524ba0c7",
+      "git-tree": "f2d5b828df130d8b1106aa0c076eb9cb79e50ea9",
       "version": "2020.0.0",
       "port-version": 2
     },

[NancyLi1013]

Probably:
https://software.intel.com/content/www/us/en/develop/tools/oneapi/base-toolkit/download.html?operatingsystem=window&distributions=webdownload&options=offline

Thanks for your suggestion. @Neumann-A

Since https://registrationcenter.intel.com/en/products/download/3178/ is also used in portfile.cmake, it would be better to keep the consistent with this in vcpkg.json.

[BillyONeal]

Thanks for your contribution!