workflows: declare empty BUILD_OPTS

While reading the workflow files without previous knowledge the use BUILD_OPTS variable is confusing, it's not clear where it comes from, or if it even is populated, this makes it clear it is just a statically empty variable for now, and can be populated if needed in the future

.github/workflows/build-linux.yml

@@ -30,6 +30,8 @@ jobs:
           - 'gcc-12'
           - 'gcc-13'
       fail-fast: false
+    env:
+      BUILD_OPTS: ''
     steps:
       - name: Runtime environment
         shell: bash

.github/workflows/build-macos.yml

@@ -105,6 +105,8 @@ jobs:
           - gcc@12
           - gcc@13 # needs hardcoding for the GCOV replacement
       fail-fast: false
+    env:
+      BUILD_OPTS: ''
     steps:
       - name: Runtime environment
         env:

.github/workflows/build-windows.yml

@@ -29,6 +29,7 @@ jobs:
       CC: cl.exe
       CXX: cl.exe
       LD: link.exe
+      BUILD_OPTS: ''
     steps:
       - name: Runtime environment
         env:
@@ -111,6 +112,8 @@ jobs:
           - ucrt64
           - clang64
       fail-fast: false
+    env:
+      BUILD_OPTS: ''
     steps:
       - name: Use MinGW from MSYS
         uses: msys2/setup-msys2@v2
@@ -154,7 +157,7 @@ jobs:
           meson --version
           ninja --version
       - name: Configure
-        run: meson build --prefix=$HOME/.local
+        run: meson build --prefix=$HOME/.local $BUILD_OPTS
       - name: Build
         run: meson compile -C build
       - name: Test