fix: rename Boost_INCLUDE_DIR to Boost_INCLUDE_DIRS

cmake sets Boost_INCLUDE_DIRS but not Boost_INCLUDE_DIR.
This fixes build issues where cmake complains that Boost_INCLUDE_DIR
is set to NOTFOUND
2 jobs for !38 with fix_boost_include_dirs in 74 minutes and 53 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #75817
build_clazy_clang_tidy

00:29:10

failed #75816
linux kf5-qt5 qt5.12

00:45:43

1.0%
 
Name Stage Failure
failed
linux kf5-qt5 qt5.12 Build
build: found 6392 matching files and directories   
WARNING: Uploading artifacts as "archive" to coordinator... failed id=75816 responseStatus=502 Bad Gateway status=502 token=isxBhTCM
WARNING: Retrying... context=artifacts-uploader error=invalid argument
WARNING: Uploading artifacts as "archive" to coordinator... failed id=75816 responseStatus=502 Bad Gateway status=502 token=isxBhTCM
WARNING: Retrying... context=artifacts-uploader error=invalid argument
WARNING: Uploading artifacts as "archive" to coordinator... failed id=75816 responseStatus=502 Bad Gateway status=502 token=isxBhTCM
FATAL: invalid argument
Cleaning up file based variables
ERROR: Job failed: exit code 1