Fix coreutils build failure and builder.py crash

[google-labs-jules]

This change fixes two critical issues:

1. Builder Crash: Resolved an AttributeError in scripts/repoexe/builder.py where self.sysroot was being accessed but not defined. Replaced it with self.get_sysroot(arch) to correctly handle architecture-specific sysroots during dependency installation.
2. Coreutils Build Failure: Fixed a build failure in coreutils where sed could not find lib/posixtm.c because it was running in the build directory. Updated the path to use $TERMUX_PKG_SRCDIR.

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PR created automatically by Jules for task 9475754221987515599 started by @SjnExe

[google-labs-jules]

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