Tests: Update PLplot contract test repository URL

Use a `git://` protocol URL documented by the repository hosting
infrastructure.  After some updates to that infrastructure, the
`https://` URL with the `.git` extension no longer works reliably.

Tests/Contracts/PLplot/CMakeLists.txt

@@ -9,7 +9,7 @@ if(NOT PLplot_GIT_TAG)
   set(PLplot_GIT_TAG "plplot-5.13.0")
 endif()
 ExternalProject_Add(PLplot
-  GIT_REPOSITORY "https://git.code.sf.net/p/plplot/plplot.git"
+  GIT_REPOSITORY "git://git.code.sf.net/p/plplot/plplot"
   GIT_TAG "${PLplot_GIT_TAG}"
   PREFIX "${PLplot_PREFIX}"
   CMAKE_ARGS