perf(exec): improve performance

...by only parsing installed packages for their environment variables if
we are in a deactivated hermit environment. When activated, the parent
shell will have already passed the new process all environment
variables, including packages that were installed after initial
activation. Parsing the HCL configs of all installed packages introduces
some overhead. This can be non-negligible when hermit-managed
packages are used a large number of times - for instance with javac or
rustc.

env.go

@@ -783,10 +783,21 @@ func (e *Env) Exec(l *ui.UI, pkg *manifest.Package, binary string, args []string
 		return errors.WithStack(err)
 	}
 
-	installed, err := e.ListInstalled(l)
-	if err != nil {
-		return errors.WithStack(err)
+	var installed []*manifest.Package
+
+	// If we are activated, the parent shell already has
+	// already passed us all necessary environment variables.
+	// This may not be the case if the user manually ran
+	// e.g. ./bin/go
+	// We still need to call e.allEnvarOpsForPackages to ensure
+	// the selected package's env vars take precedence.
+	if os.Getenv("DEACTIVATED_HERMIT") != "" {
+		installed, err = e.ListInstalled(l)
+		if err != nil {
+			return errors.WithStack(err)
+		}
 	}
+
 	ops := e.allEnvarOpsForPackages(runtimeDeps, pkg, installed...)
 	packageHermitBin, err := e.getPackageRuntimeEnvops(pkg)
 	if err != nil {
@@ -797,9 +808,6 @@ func (e *Env) Exec(l *ui.UI, pkg *manifest.Package, binary string, args []string
 	}
 	env := e.envarsFromOps(true, ops)
 
-	if err != nil {
-		return errors.WithStack(err)
-	}
 	for _, bin := range binaries {
 		if filepath.Base(bin) != filepath.Base(binary) {
 			continue