bpo-34177: vsts: Avoid conflict with Homebrew Python (GH-8430)

/usr/local/lib/pythonX.Y is used by Homebrew's Python already.
(cherry picked from commit 3e7d18a)

Author: methane

.vsts/macos-buildbot.yml

@@ -24,7 +24,7 @@ steps:
   clean: true
   fetchDepth: 5
 
-- script: ./configure --with-pydebug --with-openssl=/usr/local/opt/openssl
+- script: ./configure --with-pydebug --with-openssl=/usr/local/opt/openssl --prefix=/opt/python-vsts
   displayName: 'Configure CPython (debug)'
 
 - script: make -s -j4

.vsts/macos-pr.yml

@@ -24,7 +24,7 @@ steps:
   clean: true
   fetchDepth: 5
 
-- script: ./configure --with-pydebug --with-openssl=/usr/local/opt/openssl
+- script: ./configure --with-pydebug --with-openssl=/usr/local/opt/openssl --prefix=/opt/python-vsts
   displayName: 'Configure CPython (debug)'
 
 - script: make -s -j4