Stop LOG_DIR changes causing bp2build to rerun

Retrieving an environment var through Config.Getenv() automatically
adds it to a soong.environment.used* file along with its current value.
Changes to its current value will trigger a rebuild of the appropriate
target. The LOG_DIR is a sub directory of DIST_DIR and on build servers
DIST_DIR encodes the build number making it unique every time. So,
using Config.Getenv("LOG_DIR") will mean the CI builds will never
re-use the previous output.

Previously, bp2build and symlink forrest both added LOG_DIR to their
respective soong.environment.used* file. This change avoids that by
passing through the value from the main() function which retrieves it
using availableEnv["LOG_DIR"] which avoids it being added to that
file.

Bug: 257590265
Test: grep LOG_DIR out/soong/soong.environment.used*
      # Before this change it includes LOG_DIR, after it does not.
Change-Id: Iac76aca2d5dc04f47b0924ab5afedef5014bc8b6