iOS TestFlight: macOS base64 wants --decode, not -d

macOS uses BSD base64 where -d historically means *debug*, not
*decode* like GNU base64. On the Mac runner that bit us — the
first base64 -d invocation ran in debug mode, consumed no stdin,
and produced an empty .p12 file, then the keychain import bombed
with the now-classic "(null): error decoding base64 input stream"
once a downstream tool tried to read the empty file.

Switch all three secret decodes (.p12 cert, .mobileprovision,
.p8 API key) to base64 --decode. Long form is identical between
GNU coreutils and BSD base64.

The Apple secrets themselves were fine — sanity-check step
confirmed all non-zero lengths (TEAM_ID/KEY_ID showed as ***0
in the log, which is the actual literal 10 with the 1 redacted
because some other secret value contains the substring "1").

.github/workflows/build-ios-testflight.yml

@@ -258,7 +258,7 @@ jobs:
           security list-keychains -d user -s "$KEYCHAIN_PATH" $(security list-keychains -d user | sed 's/"//g')
           # Import the .p12
           P12="$RUNNER_TEMP/dist.p12"
-          echo "$APPLE_DIST_CERT_P12_BASE64" | base64 -d > "$P12"
+          echo "$APPLE_DIST_CERT_P12_BASE64" | base64 --decode > "$P12"
           security import "$P12" -k "$KEYCHAIN_PATH" -P "$APPLE_DIST_CERT_P12_PASSWORD" \
             -T /usr/bin/codesign -T /usr/bin/security
           security set-key-partition-list -S apple-tool:,apple:,codesign: -s -k "$APPLE_KEYCHAIN_PASSWORD" "$KEYCHAIN_PATH"
@@ -274,7 +274,7 @@ jobs:
           PROFILES_DIR="$HOME/Library/MobileDevice/Provisioning Profiles"
           mkdir -p "$PROFILES_DIR"
           PROFILE="$RUNNER_TEMP/Hash_Wallet.mobileprovision"
-          echo "$APPLE_PROVISIONING_PROFILE_BASE64" | base64 -d > "$PROFILE"
+          echo "$APPLE_PROVISIONING_PROFILE_BASE64" | base64 --decode > "$PROFILE"
 
           # Parse the .mobileprovision to get its UUID + Name (CMS-decoded plist).
           PROFILE_UUID=$(security cms -D -i "$PROFILE" | plutil -extract UUID raw -o - -)
@@ -295,7 +295,7 @@ jobs:
         run: |
           set -e
           mkdir -p "$HOME/.appstoreconnect/private_keys"
-          echo "$APPLE_API_KEY_P8_BASE64" | base64 -d > "$HOME/.appstoreconnect/private_keys/AuthKey_${APPLE_KEY_ID}.p8"
+          echo "$APPLE_API_KEY_P8_BASE64" | base64 --decode > "$HOME/.appstoreconnect/private_keys/AuthKey_${APPLE_KEY_ID}.p8"
           ls -la "$HOME/.appstoreconnect/private_keys/"
 
       # ---- Flutter init + codegen + pod install ---------------------------