Unset DYLD_INSERT_BINARIES when unneeded

Previously, the tracer environment variables were set for the
current process, and for future steps, in the init action. In
certain scenarios (such as on MacOS ARM runners with System
Integrity Protection disabled), these environment variables are
not unset by the system. In particular, the `DYLD_INSERT_BINARIES`
variable interferes with later system calls.

This change unsets the `DYLD_INSERT_BINARIES` variable for the
current process in init. It also unsets the variables either at the
beginning of autobuild, or analyze, if autobuild has not run.

Author: angelapwen

CHANGELOG.md

@@ -7,6 +7,7 @@ Note that the only difference between `v2` and `v3` of the CodeQL Action is the
 ## [UNRELEASED]
 
 - Update default CodeQL bundle version to 2.18.2. [#2417](https://github.com/github/codeql-action/pull/2417)
+- Fix a bug where system calls in the Action, such as `df`, would fail on ARM machines with System Integrity Protection disabled due to injected build tracer environment variables. [#2428](https://github.com/github/codeql-action/pull/2428)
 
 ## 3.26.1 - 13 Aug 2024