BaseTools: Edk2ToolsBuild: Fixing pipeline build due to path too long

[BobCF]

Current implementation of looking up toolchain will insert the findings from vsvarsall.bat to existing path and potentially stuff the variable to exceed the length of maximal path length accepted by Windows.

This change updated the logic to use the discovered shell varialbes to replace the existing path, which is desirable in the specific use case.

Cc: Bob Feng [email protected] Cc: Liming Gao [email protected] Cc: Yuwei Chen [email protected]

Co-authored-by: Sean Brogan [email protected] Signed-off-by: Kun Qin [email protected] Reviewed-by: Sean Brogan [email protected] Reviewed-by: Bob Feng [email protected]