fix shellcheck warnings in bash scripts

scripts/sdist-sign

@@ -17,4 +17,4 @@ python setup.py sdist
 
 D=dist/borgbackup-$R.tar.gz
 
-$GPG --detach-sign --local-user "Thomas Waldmann" --armor --output $D.asc $D
+$GPG --detach-sign --local-user "Thomas Waldmann" --armor --output "$D.asc" "$D"

scripts/sign-binaries

@@ -14,7 +14,7 @@ else
 fi
 
 for file in dist/borg-*; do
-    $GPG --local-user "Thomas Waldmann" --armor --detach-sign --output $file.asc $file
+    $GPG --local-user "Thomas Waldmann" --armor --detach-sign --output "$file.asc" "$file"
 done
 
-touch -t $D dist/*
+touch -t "$D" dist/*

scripts/upload-pypi

@@ -15,4 +15,4 @@ fi
 
 D=dist/borgbackup-$R.tar.gz
 
-twine upload $D.asc $D
+twine upload "$D.asc" "$D"