rstudio: Bump version to 1.2.5042

[PromyLOPh]

Adds R 4.0 compatibility, see rstudio/rstudio@v1.2.5033...v1.2.5042

Build currently fail due to:

/tmp/guix-build-rstudio-server-1.2.5042.drv-0/rstudio-server-1.2.5042-checkout/src/cpp/r/session/REmbeddedPosix.cpp
: In function ‘void rstudio::r::session::runEmbeddedR(const rstudio::core::FilePath&, const rstudio::core::FilePath
&, bool, bool, SA_TYPE, const rstudio::r::session::Callbacks&, rstudio::r::session::InternalCallbacks*)’:
/tmp/guix-build-rstudio-server-1.2.5042.drv-0/rstudio-server-1.2.5042-checkout/src/cpp/r/session/REmbeddedPosix.cpp
:107:8: error: ‘struct structRstart’ has no member named ‘R_Slave’
    Rp->R_Slave = FALSE ;
        ^~~~~~~
make[2]: *** [src/cpp/r/CMakeFiles/rstudio-r.dir/build.make:547: src/cpp/r/CMakeFiles/rstudio-r.dir/session/REmbedd
edPosix.cpp.o] Error 1