coverity: check out clean sources every time

Change-Id: I76782099e801ab89202c0103f97b7142b2c115fd

coverity/build_Osmocom.sh

@@ -15,10 +15,6 @@ git_branch() {
 }
 
 do_build() {
-	git fetch
-	git reset --hard "origin/$(git_branch)"
-	git clean -dxf
-	git checkout .
 	autoreconf --install --force
 	./configure --prefix="$prefix" $*
 

coverity/coverity_Osmocom.sh

@@ -6,6 +6,9 @@ base_dir="$PWD"
 src_dir="$base_dir/source-Osmocom"
 cov_dir="$src_dir/cov-int"
 
+rm -rf "$src_dir"
+./prepare_source_Osmcocom.sh
+
 export PATH="$base_dir/cov-analysis-linux64-8.5.0/bin/:$PATH"
 
 rm -rf "$cov_dir"