ci: bump shfmt to 3.5.1, simplify CI setup

1. Bump shfmt to v3.5.1. Release notes:
   https://github.com/mvdan/sh/releases

2. Since shfmt v3.5.0, specifying -l bash (or -l bats) is no longer
   necessary. Therefore, we can use shfmt to find all the files.
   Add .editorconfig to ignore vendor subdirectory.

3. Use shfmt docker image, so that we don't have to install anything
   explicitly. This greatly simplifies the shfmt CI job. Add
   localshfmt target so developers can still use a local shfmt binary
   when necessary.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -67,22 +67,6 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
-    - name: vars
-      run: |
-        echo "VERSION=3.3.1" >> $GITHUB_ENV
-        echo "$(go env GOPATH)/bin" >> $GITHUB_PATH
-    - name: cache go mod and $GOCACHE
-      uses: actions/cache@v3
-      with:
-        path: |
-          ~/go/pkg/mod
-          ~/.cache/go-build
-        key: ${{ runner.os }}-shfmt-${{ env.VERSION }}
-        restore-keys: ${{ runner.os }}-shfmt-
-    - name: install shfmt
-      run: |
-        command -v shfmt || \
-          (cd ~ && GO111MODULE=on time go get mvdan.cc/sh/v3/cmd/shfmt@v$VERSION)
     - name: shfmt
       run: make shfmt