ci: Fix macos brew install

The macos build's "brew install" command has developed a
build failure without us being involved at all:

    Error: The `brew link` step did not complete successfully
    The formula built, but is not symlinked into /opt/homebrew
    Could not symlink bin/pkg-config
    Target /opt/homebrew/bin/pkg-config
    is a symlink belonging to [email protected]. You can unlink it:
      brew unlink [email protected]

This makes the macos build work again and implements the suggestions
brew prints when installing bison and flex, regarding PATH, LDFLAGS,
and CPPFLAGS.

.github/workflows/build.yml

@@ -218,10 +218,12 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v3
+      - name: Original PATH and build tools
+        run: |
+          echo "PATH=$PATH"
+          for b in bison flex pkg-config; do type "$b"; done
       - name: Install prerequisites
         run: >-
-          # brew update
-
           brew install
           cmake
           flex
@@ -233,12 +235,18 @@ jobs:
           libftdi
           readline
           libserialport
-          pkg-config
+      - name: Configure environment to use brew kegs for flex and bison
+        run: |
+          echo "PATH=/opt/homebrew/opt/flex/bin:/opt/homebrew/opt/bison/bin:$PATH" >> $GITHUB_ENV
+      - name: post-brew PATH and build tools
+        run: |
+          echo "PATH=$PATH"
+          for b in bison flex pkg-config; do type "$b"; done
       - name: Configure
         run: >-
           cmake
-          -D CMAKE_C_FLAGS=-I/opt/homebrew/include
-          -D CMAKE_EXE_LINKER_FLAGS=-L/opt/homebrew/Cellar
+          -D CMAKE_C_FLAGS="-I/opt/homebrew/include -I/opt/homebrew/opt/flex/include"
+          -D CMAKE_EXE_LINKER_FLAGS="-L/opt/homebrew/Cellar -L/opt/homebew/opt/flex/lib"
           -D DEBUG_CMAKE=1
           -D CMAKE_BUILD_TYPE=${{env.BUILD_TYPE}}
           -B build
@@ -269,10 +277,12 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v3
+      - name: Original PATH and build tools
+        run: |
+          echo "PATH=$PATH"
+          for b in bison flex pkg-config; do type "$b"; done
       - name: Install prerequisites
         run: >-
-          # brew update
-
           brew install
           automake
           autoconf
@@ -286,15 +296,21 @@ jobs:
           libftdi
           readline
           libserialport
-          pkg-config
+      - name: Configure environment to use brew kegs for flex and bison
+        run: |
+          echo "PATH=/opt/homebrew/opt/flex/bin:/opt/homebrew/opt/bison/bin:$PATH" >> $GITHUB_ENV
+      - name: post-brew PATH and build tools
+        run: |
+          echo "PATH=$PATH"
+          for b in bison flex pkg-config; do type "$b"; done
       - name: Configure
         run: >-
           ./src/bootstrap
 
           mkdir _ambuild && cd _ambuild
 
-          CFLAGS="-I/opt/homebrew/include"
-          LDFLAGS="-L/opt/homebrew/lib"
+          CFLAGS="-I/opt/homebrew/include -I/opt/homebrew/opt/flex/include"
+          LDFLAGS="-L/opt/homebrew/lib    -L/opt/homebrew/opt/bison/lib"
           ../src/configure
 
       - name: Build