[continuous-integration] remove git clean in scripts (#4988)

This removes the use of `git clean` in scripts to prevent accidental
loss of work.

script/check-simulation-build-cmake

@@ -29,6 +29,9 @@
 
 set -euxo pipefail
 
+readonly OT_SRCDIR="$(pwd)"
+readonly OT_BUILDDIR="$OT_SRCDIR/build"
+
 readonly OT_COMMON_OPTIONS=(
     "-DOT_COMPILE_WARNING_AS_ERROR=on"
     "-DOT_PLATFORM=simulation"
@@ -36,8 +39,7 @@ readonly OT_COMMON_OPTIONS=(
 
 reset_source()
 {
-    git checkout -- .
-    git clean -xfd
+    rm -rf "$OT_BUILDDIR"
 }
 
 build_all_features()
@@ -71,21 +73,20 @@ build_all_features()
 
     reset_source
 
-    git checkout -- .
-    git clean -xfd
-    mkdir build && cd build
+    mkdir -p "$OT_BUILDDIR"
+    cd "$OT_BUILDDIR"
     cmake -GNinja "${options[@]}" ..
     ninja
-    cd ..
+    cd "$OT_SRCDIR"
 }
 
 build_toranj()
 {
     reset_source
-    ./tests/toranj/build.sh cmake
+    top_builddir="$OT_BUILDDIR" ./tests/toranj/build.sh cmake
 
     reset_source
-    ./tests/toranj/build.sh cmake-posix
+    top_builddir="$OT_BUILDDIR" ./tests/toranj/build.sh cmake-posix
 }
 
 main()