fix(CI): use 1.53.0 rust image instead of 1.43 and fix example

fix test string_vars to never parse input of `ls` and never delete the
created folders, because deletion breaks CI for unclear reasons.

.gitlab-ci.yml

@@ -19,7 +19,7 @@ format:
     - cargo +nightly fmt --all -- --check
 
 linux:
-  image: 'rust:1.43.0'
+  image: 'rust:1.53.0'
   cache:
     key: linux
     paths:

tests/string_vars.ion

 echo '# ANCHOR: string_variables'
+# The CI can not handle deletions properly.
 mkdir -p _tmp _tmp/t1 _tmp/t2
 cd _tmp
-let filelist = $(ls ^> /dev/null)
+let filelist = *
 echo $filelist
 cd ..
-rm -fr _tmp
 echo '# ANCHOR_END: string_variables'
 echo '# ANCHOR: string_slicing'
 let foo = "Hello, World"

tests/string_vars.out

 # ANCHOR: string_variables
-t1
-t2
+t1 t2
 # ANCHOR_END: string_variables
 # ANCHOR: string_slicing
 Hello