Merge bitcoin/bitcoin#22075: guix: Misc leftover usability improvements

108a6be92a guix: Check for disk space availability before building (Carl Dong)
d7dec89091 guix: Remove dest if OUTDIR mv fails (Carl Dong)

Pull request description:

  There seems to be some corner cases that can be hit when guix scripts unexpectedly fail in the middle of operation, see: https://gnusha.org/bitcoin-builds/2021-05-24.log

  - Perform an early disk space check for `guix-build`
  - Overwrite existing output directory after a successful build (the existing one might be malformed), and cleanup output directory if the `mv` somehow fails

ACKs for top commit:
  laanwj:
    Tested ACK 108a6be92a
  achow101:
    ACK 108a6be92a

Tree-SHA512: cf6438317da40bf55714cd2d8cce859b3d435cc66cabefe8d4a53552d7880966acfe84ffe8fadf1c80e368ae6b037992258a6d409df85ffc6ce8bf780e98e2e5

contrib/guix/libexec/build.sh

@@ -447,4 +447,6 @@ mkdir -p "$DISTSRC"
     esac
 )  # $DISTSRC
 
-mv --no-target-directory "$OUTDIR" "$ACTUAL_OUTDIR"
+rm -rf "$ACTUAL_OUTDIR"
+mv --no-target-directory "$OUTDIR" "$ACTUAL_OUTDIR" \
+    || ( rm -rf "$ACTUAL_OUTDIR" && exit 1 )

contrib/guix/libexec/codesign.sh

@@ -100,4 +100,6 @@ mkdir -p "$DISTSRC"
     esac
 )  # $DISTSRC
 
-mv --no-target-directory "$OUTDIR" "$ACTUAL_OUTDIR"
+rm -rf "$ACTUAL_OUTDIR"
+mv --no-target-directory "$OUTDIR" "$ACTUAL_OUTDIR" \
+    || ( rm -rf "$ACTUAL_OUTDIR" && exit 1 )