fix(build): Run plugin script directly instead of with sh (#2025)

These github actions run on Ubuntu based machine. Ubuntu by default
links `/bin/sh` to dash. So by invoking the plugin.sh script as `/bin/sh
./plugins.sh` the script would run under `dash` despite the shebang
line. A recent change that unified some of the build process, introduced
additional bashisms into this script which is caused the build to fail.

This updated the build system to just run the script directly, which
will honor the shebang line. This seems like the best approach since the
ubuntu machine has bash and the shebang line points to bash, so it is
natural for developers to assume bashisms will work.

See:
    f370abc1da
    https://github.com/actions/virtual-environments/blob/ubuntu20/20220410.2/images/linux/Ubuntu2004-Readme.md
    https://wiki.ubuntu.com/DashAsBinSh

.github/workflows/build.yml

@@ -124,7 +124,7 @@ jobs:
           mkdir -p dist out
           unset GOPATH;
           # Build plugins
-          sh ./scripts/plugins.sh
+          ./scripts/plugins.sh
           go build -v -tags "ui" -ldflags "${{ env.LD_FLAGS }}" -o dist/ ./cmd/boundary
           zip -r -j out/${{ env.PKG_NAME }}_${{ needs.get-product-version.outputs.product-version }}_${{ matrix.goos }}_${{ matrix.goarch }}.zip dist/
       - uses: actions/upload-artifact@v2
@@ -190,7 +190,7 @@ jobs:
           mkdir -p dist out
           unset GOPATH;
           # Build plugins
-          sh ./scripts/plugins.sh
+          ./scripts/plugins.sh
           go build -v -tags "ui" -ldflags "${{ env.LD_FLAGS }}" -o dist/ ./cmd/boundary
           zip -r -j out/${{ env.PKG_NAME }}_${{ needs.get-product-version.outputs.product-version }}_${{ matrix.goos }}_${{ matrix.goarch }}.zip dist/
       - uses: actions/upload-artifact@v2
@@ -283,7 +283,7 @@ jobs:
           mkdir -p dist out
           unset GOPATH;
           # Build plugins
-          sh ./scripts/plugins.sh
+          ./scripts/plugins.sh
           go build -v -tags "ui netcgo" -ldflags "${{ env.LD_FLAGS }}" -o dist/ ./cmd/boundary
           zip -r -j out/${{ env.PKG_NAME }}_${{ needs.get-product-version.outputs.product-version }}_${{ matrix.goos }}_${{ matrix.goarch }}.zip dist/
       - uses: actions/upload-artifact@v2