[cmake] Use FetchContent_MakeAvailable

[crueter]

Populate is deprecated, and in fact appears to completely not work on GitHub's
Windows runner, not sure why, but I confirmed that this fixed it.

Signed-off-by: crueter [email protected]

[crueter]

Okay, that's interesting, it DIDN'T actually fix Windows, it's a different error:

[crueter]

Huh, I can't reproduce this at all on a local machine:

C:\Users\Administrator\code\quazip>cmake --preset vcpkg -B build -DCMAKE_PREFIX_PATH=C:\Qt\6.9.0\msvc2022_64 -DFETCHCONTENT_QUIET=OFF
...
-- Fetching bzip2 https://sourceware.org/git/bzip2.git master
-- Populating bzip2
-- Configuring done (0.1s)
-- Generating done (0.0s)
-- Build files have been written to: C:/Users/Administrator/code/quazip/build/_deps/bzip2-subbuild
[ 11%] Creating directories for 'bzip2-populate'
[ 22%] Performing download step (git clone) for 'bzip2-populate'
Cloning into 'bzip2'...
Your branch is up to date with 'origin/master'.
Already on 'master'
[ 33%] Performing update step for 'bzip2-populate'
-- Fetching latest from the remote origin
[ 44%] No patch step for 'bzip2-populate'
[ 55%] No configure step for 'bzip2-populate'
[ 66%] No build step for 'bzip2-populate'
[ 77%] No install step for 'bzip2-populate'
[ 88%] No test step for 'bzip2-populate'
[100%] Completed 'bzip2-populate'
[100%] Built target bzip2-populate
-- Could NOT find BZip2 (missing: BZIP2_LIBRARIES BZIP2_INCLUDE_DIR)
-- Configuring done (5.2s)
-- Generating done (0.1s)
-- Build files have been written to: C:/Users/Administrator/code/quazip/build