Remove duplicated list BUILD, derive one from the other

[dsengupta0628]

Long term maintainance for this fix: #391

spice/WriteSpice.tcl was never added to the Tcl lists in src/sta/BUILD, though CMake has it in both its encode lists (my slip while pulling upstream).
The SWIG glue (WriteSpice.i) is listed, so the C++ compiled - the Tcl command wrappers just never get registered.

The deeper issue though: src/sta/BUILD maintains two Tcl lists by hand.
tcl_srcs : encoded into the standalone OpenSTA binary
exported_tcl : re-exported via dbSta into the OpenROAD binary

exported_tcl is just tcl_srcs minus a few files OpenROAD replaces (Init.tcl, network/Link.tcl, verilog/Verilog.tcl).
Keeping two parallel lists in sync manually is kind of risky.

Potential fix for this long term- that will derive exported_tcl from tcl_srcs instead of maintaining it separately:

  sta_only_tcl = [
      "tcl/Init.tcl",
      "network/Link.tcl",
      "verilog/Verilog.tcl",
  ]
  exported_tcl = [f for f in tcl_srcs if f not in sta_only_tcl]

This makes tcl_srcs the single source of truth; any new command file flows to both binaries automatically unless explicitly excluded.
Set is identical to today; only ordering changes, which is safe (the two build systems already use different orders and both work).

[gemini-code-assist]

Code Review

This pull request updates the BUILD file to dynamically derive the exported_tcl list from tcl_srcs by excluding specific standalone OpenSTA shell files (sta_only_tcl). This establishes tcl_srcs as the single source of truth and avoids manual maintenance of a duplicate list. There are no review comments, and I have no additional feedback to provide.