espressif: Improve CI script for targetting multiple chips at once

This enables using "espressif_run.sh" locally for a quick validation
that the build passes for multiple Espressif targets.

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>
diff --git a/.github/workflows/espressif.yaml b/.github/workflows/espressif.yaml
index ada3067..09870f5 100644
--- a/.github/workflows/espressif.yaml
+++ b/.github/workflows/espressif.yaml
@@ -23,7 +23,7 @@
         - "sign-rsa2048,sign-rsa3072,sign-ec256,sign-ed25519"
     runs-on: ubuntu-latest
     env:
-      MCUBOOT_TARGET: ${{ matrix.targets }}
+      MCUBOOT_TARGETS: ${{ matrix.targets }}
       MCUBOOT_FEATURES: ${{ matrix.features }}
     steps:
     - uses: actions/checkout@v2