Use more robust check for sed version.

Generally, macOS uses BSD sed and Linux uses GNU sed. However, in some circumstances (CI macOS arm64) it appears that macOS may be using GNU instead of BSD. Therefore it is best to check sed directly for BSD/GNU rather than using the platform as a proxy.

Remove patch for old-style markers.

installers-conda/resources/post-install.sh

@@ -15,10 +15,8 @@ spy_exe=${PREFIX}/envs/spyder-runtime/bin/spyder
 u_spy_exe=${PREFIX}/uninstall-spyder.sh
 all_user=$([[ -e ${PREFIX}/.nonadmin ]] && echo false || echo true)
 
-sed_opts=("-i")
 alias_text="alias uninstall-spyder=${u_spy_exe}"
 if [[ "$OSTYPE" = "darwin"* ]]; then
-    sed_opts+=("", "-e")
     shortcut_path="/Applications/${INSTALLER_NAME}.app"
     if [[ "$all_user" = "false" ]]; then
         shortcut_path="${HOME}${shortcut_path}"
@@ -36,6 +34,16 @@ else
     fi
 fi
 
+
+# BSD sed requires extra "" after -i flag
+if [[ $(sed --version 2>/dev/null) ]]; then
+    # GNU sed has --version
+    sed_opts=("-i" "-e")
+else
+    # BSD sed does not have --version
+    sed_opts=("-i" "" "-e")
+fi
+
 m1="# >>> Added by Spyder >>>"
 m2="# <<< Added by Spyder <<<"
 
@@ -45,12 +53,7 @@ add_alias() {
         exit 0
     fi
 
-    # Remove old-style markers, if present; discard after EXPERIMENTAL
-    # installer attrition.
-    sed ${sed_opts[@]} "/# <<<< Added by Spyder <<<</,/# >>>> Added by Spyder >>>>/d" $shell_init
-
-    # Posix compliant sed does not like semicolons.
-    # Must use newlines to work on macOS
+    # BSD sed does not like semicolons; newlines work for both BSD and GNU.
     sed ${sed_opts[@]} "
     /$m1/,/$m2/{
         h