ci: Rename `TIDY_LLVM_V` to `IWYU_LLVM_V` in IWYU-specific code

diff --git a/ci/test/00_setup_env_native_iwyu.sh b/ci/test/00_setup_env_native_iwyu.sh
index b801b9ac..27b1e02b 100755
--- a/ci/test/00_setup_env_native_iwyu.sh
+++ b/ci/test/00_setup_env_native_iwyu.sh
@@ -8,9 +8,9 @@ export LC_ALL=C.UTF-8
 
 export CI_IMAGE_NAME_TAG="mirror.gcr.io/debian:trixie"  # To build codegen, CMake must be 3.31 or newer.
 export CONTAINER_NAME=ci_native_iwyu
-export TIDY_LLVM_V="22"
-export APT_LLVM_V="${TIDY_LLVM_V}"
-export PACKAGES="clang-${TIDY_LLVM_V} clang-format-${TIDY_LLVM_V} libclang-${TIDY_LLVM_V}-dev llvm-${TIDY_LLVM_V}-dev jq libevent-dev libboost-dev libzmq3-dev systemtap-sdt-dev qt6-base-dev qt6-tools-dev qt6-l10n-tools libqrencode-dev libsqlite3-dev libcapnp-dev capnproto"
+export IWYU_LLVM_V="22"
+export APT_LLVM_V="${IWYU_LLVM_V}"
+export PACKAGES="clang-${IWYU_LLVM_V} clang-format-${IWYU_LLVM_V} libclang-${IWYU_LLVM_V}-dev llvm-${IWYU_LLVM_V}-dev jq libevent-dev libboost-dev libzmq3-dev systemtap-sdt-dev qt6-base-dev qt6-tools-dev qt6-l10n-tools libqrencode-dev libsqlite3-dev libcapnp-dev capnproto"
 export NO_DEPENDS=1
 export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
@@ -20,6 +20,6 @@ export RUN_IWYU=true
 export GOAL="codegen"
 export BITCOIN_CONFIG="\
  --preset dev-mode -DBUILD_GUI=OFF \
- -DCMAKE_C_COMPILER=clang-${TIDY_LLVM_V} \
- -DCMAKE_CXX_COMPILER=clang++-${TIDY_LLVM_V} \
+ -DCMAKE_C_COMPILER=clang-${IWYU_LLVM_V} \
+ -DCMAKE_CXX_COMPILER=clang++-${IWYU_LLVM_V} \
 "
diff --git a/ci/test/01_base_install.sh b/ci/test/01_base_install.sh
index 308f7abe..c1908b7c 100755
--- a/ci/test/01_base_install.sh
+++ b/ci/test/01_base_install.sh
@@ -85,9 +85,9 @@ if [[ -n "${USE_INSTRUMENTED_LIBCPP}" ]]; then
 fi
 
 if [[ "${RUN_IWYU}" == true ]]; then
-  ${CI_RETRY_EXE} git clone --depth=1 https://github.com/include-what-you-use/include-what-you-use -b clang_"${TIDY_LLVM_V}" /include-what-you-use
+  ${CI_RETRY_EXE} git clone --depth=1 https://github.com/include-what-you-use/include-what-you-use -b clang_"${IWYU_LLVM_V}" /include-what-you-use
   (cd /include-what-you-use && patch -p1 < /ci_container_base/ci/test/01_iwyu.patch)
-  cmake -B /iwyu-build/ -G 'Unix Makefiles' -DCMAKE_PREFIX_PATH=/usr/lib/llvm-"${TIDY_LLVM_V}" -S /include-what-you-use
+  cmake -B /iwyu-build/ -G 'Unix Makefiles' -DCMAKE_PREFIX_PATH=/usr/lib/llvm-"${IWYU_LLVM_V}" -S /include-what-you-use
   make -C /iwyu-build/ install "$MAKEJOBS"
 fi
 
diff --git a/ci/test/03_test_script.sh b/ci/test/03_test_script.sh
index bfa733b0..7b4d7837 100755
--- a/ci/test/03_test_script.sh
+++ b/ci/test/03_test_script.sh
@@ -224,7 +224,7 @@ if [[ "${RUN_IWYU}" == true ]]; then
              -Xiwyu --check_also="*/primitives/*.h" \
              2>&1 | tee /tmp/iwyu_ci.out
     python3 "/include-what-you-use/fix_includes.py" --nosafe_headers < /tmp/iwyu_ci.out
-    git diff -U1 | ./contrib/devtools/clang-format-diff.py -binary="clang-format-${TIDY_LLVM_V}" -p1 -i -v
+    git diff -U1 | ./contrib/devtools/clang-format-diff.py -binary="clang-format-${IWYU_LLVM_V}" -p1 -i -v
   }
 
   run_iwyu "compile_commands_iwyu_errors.json"