Fix github action

Author: lululxvi

.github/workflows/build.yml

@@ -57,31 +57,31 @@ jobs:
           python -c "import deepxde"
 
       - name: Test build with Jax
-        if: matrix.os != "windows-latest"  # jaxlib is not available on windows
+        if: matrix.os != 'windows-latest'  # jaxlib is not available on windows
         env:
           DDE_BACKEND: jax
         run: |
           python -m pip install jax flax optax
           python -c "import deepxde"
 
       - name: Test build with Paddle (MacOS)
-        if: matrix.os == "macos-latest"
+        if: matrix.os == 'macos-latest'
         run: |
           python -m pip install paddlepaddle==0.0.0 -f https://www.paddlepaddle.org.cn/whl/mac/cpu-noavx/develop.html
           python -c "import deepxde"
         env:
           DDE_BACKEND: paddle
 
       - name: Test build with Paddle (Linux)
-        if: matrix.os == "ubuntu-latest"
+        if: matrix.os == 'ubuntu-latest'
         run: |
           python -m pip install paddlepaddle==0.0.0 -f https://www.paddlepaddle.org.cn/whl/linux/cpu-mkl/develop.html
           python -c "import deepxde"
         env:
           DDE_BACKEND: paddle
 
       - name: Test build with Paddle (Windows)
-        if: matrix.os == "windows-latest"
+        if: matrix.os == 'windows-latest'
         run: |
           python -m pip install paddlepaddle==0.0.0 -f https://www.paddlepaddle.org.cn/whl/windows/cpu-mkl-avx/develop.html
           python -c "import deepxde"