GH-53995: [C++] Fix FetchContent build failure on CMake < 3.30

[pratikdeoke]

Rationale for this change

When building Arrow via CMake's FetchContent on versions older than 3.30,
the variable arrow_SOURCE_DIR (lowercase) gets overridden by FetchContent
because project(arrow) sets it as a cache variable while FetchContent sets
a normal variable with higher precedence. This causes Arrow to fail finding
thirdparty/versions.txt.

What changes are included in this PR?

Replace arrow_SOURCE_DIR with ARROW_SOURCE_DIR in
cpp/cmake_modules/ThirdpartyToolchain.cmake so that Arrow does not rely
on the lowercase variable that conflicts with FetchContent on older CMake.

Are these changes tested?

The fix was verified via the workaround patch applied in
apache/iceberg-cpp#1029, which applies the same change and confirmed it
resolves the issue on CMake 3.28.3.

Fixes: #53995

• GitHub Issue: [C++] Building Arrow via FetchContent can fail with versions older than CMake 3.30 #53995

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[github-actions]

⚠️ GitHub issue #53995 has been automatically assigned in GitHub to PR creator.