Add an environment variable for the include folder in the CBMC directory.

The build requirements have failed, and any other changes are very *very*
brittle, as they require downloading a CBMC package (so that an `include/cprover/`
folder is present), knowing the version and the location of the folder, and
that will also fail on the release PRs incrementing the version (as the packaged
versions and the reported version are going to be divergent).

Author: NlightNFotis

.github/workflows/pull-request-check-rust-api.yaml

@@ -7,6 +7,7 @@ on:
 env:
   default_build_dir: "build/"
   default_solver: "minisat2"
+  default_include_dir: "src/libcprover-cpp/"
 
 # For now, we support two jobs: A Linux and a MacOS based one.
 # Both of the jobs use CMake, as we only support building the Rust
@@ -64,7 +65,7 @@ jobs:
           VERSION=$(cat src/config.inc | python3 -c "import sys,re;line = [line for line in sys.stdin if re.search('CBMC_VERSION = (\d+\.\d+\.\d+)', line)];sys.stdout.write(re.search('CBMC_VERSION = (\d+\.\d+\.\d+)', line[0]).group(1))")
           cd src/libcprover-rust;\
           cargo clean;\
-          CBMC_LIB_DIR=../../${{env.default_build_dir}}/lib CBMC_VERSION=$VERSION cargo test -- --test-threads=1
+          CBMC_INCLUDE_DIR=../../${{env.default_include_dir}} CBMC_LIB_DIR=../../${{env.default_build_dir}}/lib CBMC_VERSION=$VERSION cargo test -- --test-threads=1
 
 
   check-macos-12-cmake-clang-rust:
@@ -104,4 +105,4 @@ jobs:
           VERSION=$(cat src/config.inc | python3 -c "import sys,re;line = [line for line in sys.stdin if re.search('CBMC_VERSION = (\d+\.\d+\.\d+)', line)];sys.stdout.write(re.search('CBMC_VERSION = (\d+\.\d+\.\d+)', line[0]).group(1))")
           cd src/libcprover-rust;\
           cargo clean;\
-          CBMC_LIB_DIR=../../${{env.default_build_dir}}/lib CBMC_VERSION=$VERSION cargo test -- --test-threads=1
+          CBMC_INCLUDE_DIR=../../${{env.default_include_dir}} CBMC_LIB_DIR=../../${{env.default_build_dir}}/lib CBMC_VERSION=$VERSION cargo test -- --test-threads=1