clippy: always create the capture file for Clippy

... to avoid triggering a non-fatal failure while exporting files
from the chroot in case Clippy is enabled for a non-Rust project:
```
tar: /builddir/clippy-output.txt: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
```

Related: https://issues.redhat.com/browse/OSH-57

scripts/inject-clippy.sh

@@ -3,9 +3,11 @@
 ORIGINAL_LOCATION="/usr/bin/cargo"
 NEW_LOCATION="/usr/bin/cargo_original"
 
-if [ -f "$NEW_LOCATION" ]; then
-    # remove capture file created by previous runs if --skip-init is in effect
-    rm -fv "/builddir/clippy-output.txt"
+# create empty or truncate existing (if --skip-init is in effect) capture file
+runuser mockbuild -c "truncate --size=0 /builddir/clippy-output.txt"
+
+if [ -x "$NEW_LOCATION" ]; then
+    # binary already moved (most likely by a previous run of this script)
     exit 0
 fi