guix: Create windeploy inside distsrc-*

./windeploy is a "working directory", and therefore belongs inside
distsrc-*. Many people have noticed their Guix builds failing after
hours simply because they did not remove windeploy (but did remove the
distsrc-* directories).

contrib/guix/libexec/build.sh

@@ -394,15 +394,14 @@ mkdir -p "$DISTSRC"
                     || ( rm -f "${OUTDIR}/${DISTNAME}-${HOST//x86_64-apple-darwin18/osx64}.tar.gz" && exit 1 )
                 ;;
         esac
-    )
-)
+    )  # $DISTSRC/installed
 
-case "$HOST" in
+    case "$HOST" in
         *mingw*)
             cp -rf --target-directory=. contrib/windeploy
             (
                 cd ./windeploy
-            mkdir unsigned
+                mkdir -p unsigned
                 cp --target-directory=unsigned/ "${OUTDIR}/${DISTNAME}-win64-setup-unsigned.exe"
                 find . -print0 \
                     | sort --zero-terminated \
@@ -411,4 +410,5 @@ case "$HOST" in
                     || ( rm -f "${OUTDIR}/${DISTNAME}-win-unsigned.tar.gz" && exit 1 )
             )
             ;;
-esac
+    esac
+)  # $DISTSRC