Merge bitcoin/bitcoin#24520: guix: only check for the macOS SDK once

e802310 guix: only check for the macOS SDK once (fanquake)

Pull request description:

  If we are building for both macOS HOSTS, there's no need to check and
  print that the SDK exists two times.

  Currently a Guix build for both HOSTS will print:
  ```bash
  ./contrib/guix/guix-build
  Found macOS SDK at '/SDKs/Xcode-12.2-12B45b-extracted-SDK-with-libcxx-headers', using...
  Found macOS SDK at '/SDKs/Xcode-12.2-12B45b-extracted-SDK-with-libcxx-headers', using...
  Checking that we can connect to the guix-daemon...
  ```

ACKs for top commit:
  laanwj:
    Code review ACK e802310
  achow101:
    ACK e802310

Tree-SHA512: 7e9ee7793c5dc1eb485806ca3d613742397d2cc62525203a168cad1ec96aabfd4e63dc3f2e8d205bdb2f3c2079f731d75c5d162d55ff0d42a7e6f3d01d3a7db1

Author: achow101

contrib/guix/guix-build

@@ -121,7 +121,7 @@ else
 fi
 
 ################
-# When building for darwin, the macOS SDK should exists
+# When building for darwin, the macOS SDK should exist
 ################
 
 for host in $HOSTS; do
@@ -130,6 +130,7 @@ for host in $HOSTS; do
             OSX_SDK="$(make -C "${PWD}/depends" --no-print-directory HOST="$host" print-OSX_SDK | sed 's@^[^=]\+=@@g')"
             if [ -e "$OSX_SDK" ]; then
                 echo "Found macOS SDK at '${OSX_SDK}', using..."
+                break
             else
                 echo "macOS SDK does not exist at '${OSX_SDK}', please place the extracted, untarred SDK there to perform darwin builds, exiting..."
                 exit 1