Don't set shell explicitly for the build steps

Bash is used by default under all platforms we're going to support here,
so there is no need to specify it explicitly. Moreover, by default the
appropriate shell options (such as "pipefail", as well as "e") are used,
and there is no need to override them.

.github/workflows/ci.yml

@@ -63,9 +63,8 @@ jobs:
       with:
         key: ${{ matrix.os || 'ubuntu-20.04' }}-${{ matrix.SWIGLANG }}
     - name: configure
-      shell: bash
       run: |
-          set -ex
+          set -x
           export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
           source Tools/GHA-linux-install.sh
           if test -n "$CPP11"; then CONFIGOPTS+=(--enable-cpp11-testing "CXXFLAGS=-std=c++11 $CXXFLAGS" "CFLAGS=-std=c11 $CFLAGS") && export CSTD=c11 && export CPPSTD=c++11; fi
@@ -92,9 +91,8 @@ jobs:
           echo SWIGJOBS=-j$cpu_count >> $GITHUB_ENV
 
     - name: build
-      shell: bash
       run: |
-          set -ex
+          set -x
           cd ${GITHUB_WORKSPACE}/build/build;
           make -s $SWIGJOBS
           ./swig -version && ./swig -pcreversion
@@ -103,9 +101,8 @@ jobs:
           echo 'Installing...'
           if test -z "$SWIGLANG"; then sudo make -s install && swig -version && ccache-swig -V; fi
     - name: tests
-      shell: bash
       run: |
-          set -ex
+          set -x
           cd ${GITHUB_WORKSPACE}/build/build;
           if test -n "$CPP11"; then export CPPSTD=c++11; fi
           if test -n "$CPP14"; then export CPPSTD=c++14; fi