Use --entry-point not -c when building pex (pantsbuild#6349)

I deterministically get this error:

RuntimeError: Ambiguous script specification pants matches multiple entry points:pants = pants.bin.pants_loader:main pants = pants.bin.pants_loader:main

when building with -c.

pantsbuild#6267 changed this behaviour,
but seems broken - this commit allows me to successfully build pexes.

build-support/bin/release.sh

@@ -609,7 +609,7 @@ function build_pex() {
 
     ./${PEX_PEX} \
       -o "${dest}" \
-      -c pants \
+      --entry-point="pants.bin.pants_loader:main" \
       --no-build \
       --no-pypi \
       --disable-cache \