CI: install gcc 10 only in U20.04

This modifies the commit 7f166a2. Although Linux CI is run with U20.04,
Coverity workflow uses ubuntu-latest, which is currently 22.04, where
default gcc is actually 11 and this breaks GPUJPEG compilation with
error starting with:
```
-- The C compiler identification is GNU 10.5.0
CMake Error at /usr/local/share/cmake-3.28/Modules/CMakeDetermineCompilerId.cmake:780 (message):
  Compiling the CUDA compiler identification source file
  "CMakeCUDACompilerId.cu" failed.

  Compiler: /usr/bin/nvcc
```

fixes failed run <https://github.com/CESNET/UltraGrid/actions/runs/7968323068>

.github/scripts/Linux/prepare.sh

@@ -27,10 +27,15 @@ sudo apt install libcurl4-nss-dev
 sudo apt install i965-va-driver-shaders # instead of i965-va-driver
 sudo apt install uuid-dev # Cineform
 
-sudo apt install gcc-10
-sudo apt install g++-10
-sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 10
-sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 10
+(
+        . /etc/os-release
+        if [ "$ID" != ubuntu ] || [ "$VERSION_ID" != 20.04 ]; then
+                exit
+        fi
+        sudo apt install gcc-10 g++-10
+        sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 10
+        sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 10
+)
 
 get_build_deps_excl() { # $2 - pattern to exclude
         apt-cache showsrc "$1" | sed -n '/^Build-Depends:/{s/Build-Depends://;p;q}' | tr ',' '\n' | cut -f 2 -d\  | grep -v "$2"