pvscheck.sh: set --sourcetree-root [ci skip]

Reverts previous experiment. PVS root is working correctly, one can
observe this in the PVS-studio.err file, for example:

    /usr/local/clang-7.0.0/lib/clang/7.0.0/include/stddef.h:51:1: warning: V677 ...
    /usr/local/clang-7.0.0/lib/clang/7.0.0/include/stddef.h:132:1: warning: V677 ...
    ./src/nvim/fileio.c:1382:1: warning: V1026 ...
    ./src/nvim/fileio.c:1388:1: warning: V1026 ...

The "./src/nvim/…" paths are correctly rooted, yet PVS somehow still
thinks it should analyze "/usr/local/clang-7.0.0/…".

See also: https://stackoverflow.com/q/44906903

scripts/pvscheck.sh

@@ -371,7 +371,7 @@ run_analysis() {(
       --output-file PVS-studio.log \
       --verbose \
       --file build/compile_commands.json \
-    || true
+      --sourcetree-root . || true
 
   rm -rf PVS-studio.{xml,err,tsk,html.d}
   local plog_args="PVS-studio.log --srcRoot . --excludedCodes V011"