fix(pg_stat_monitor): use pkgs.pkgsLinux consistently for all package references

The pg_stat_monitor test was mixing pkgs.system and pkgs.stdenv.hostPlatform.system
which resolved to Darwin packages on macOS, but the test requires Linux packages
for NixOS VMs. Changed to consistently use pkgs.pkgsLinux.stdenv.hostPlatform.system
and pkgs.pkgsLinux.buildEnv/makeWrapper throughout, matching the pattern used in
pgmq.nix and pgsodium.nix.

Author: yvan-sraka

nix/ext/tests/pg_stat_monitor.nix

@@ -32,13 +32,13 @@ let
   inherit (pkgs) lib;
   installedExtension =
     postgresMajorVersion:
-    self.legacyPackages.${pkgs.system}."psql_${postgresMajorVersion}".exts."${pname}";
+    self.legacyPackages.${pkgs.pkgsLinux.stdenv.hostPlatform.system}."psql_${postgresMajorVersion}".exts."${pname}";
   versions = postgresqlMajorVersion: (installedExtension postgresqlMajorVersion).versions;
   postgresqlWithExtension =
     postgresql:
     let
       majorVersion = lib.versions.major postgresql.version;
-      pkg = pkgs.buildEnv {
+      pkg = pkgs.pkgsLinux.buildEnv {
         name = "postgresql-${majorVersion}-${pname}";
         paths = [
           postgresql
@@ -53,7 +53,7 @@ let
           withJIT = pkg;
           withoutJIT = pkg;
         };
-        nativeBuildInputs = [ pkgs.makeWrapper ];
+        nativeBuildInputs = [ pkgs.pkgsLinux.makeWrapper ];
         pathsToLink = [
           "/"
           "/bin"
@@ -67,8 +67,8 @@ let
       };
     in
     pkg;
-  psql_15 = postgresqlWithExtension self.packages.${pkgs.stdenv.hostPlatform.system}.postgresql_15;
-  psql_17 = postgresqlWithExtension self.packages.${pkgs.stdenv.hostPlatform.system}.postgresql_17;
+  psql_15 = postgresqlWithExtension self.packages.${pkgs.pkgsLinux.stdenv.hostPlatform.system}.postgresql_15;
+  psql_17 = postgresqlWithExtension self.packages.${pkgs.pkgsLinux.stdenv.hostPlatform.system}.postgresql_17;
 in
 pkgs.testers.runNixOSTest {
   name = pname;