fix: use SplitN when working with vars, unbreak vars with = in them

Previously, the functions that interacted with and handling the
splitting of variables into names/values was using `strings.Split()`,
which splits on each token and returns a slice with all of them.

In the context we're operating here where it's `var=val`, we only ever
need to split on the first `=`, so this switches to `strings.SplitN()`
with a count of 2. This ensures that we only ever split on the first `=`
present and that the second slice item is the entire remainder of the
unsplit string. This ensures that variables with quotes in them can be
properly parsed and not be skipped for being invalid. Ie, a var like
this will now be properly parsed:

`NODE_EXPORTER_FLAGS='--web.listen-address=":9100"'

internal/shell/shell.go

@@ -101,7 +101,7 @@ func getUpdatedVars(oldVars, newVars VariableSlice) VariableSlice {
 
 	// compare envs before and after and only return the new/updated variables
 	for _, newKV := range newVars {
-		newTokens := strings.Split(newKV, "=")
+		newTokens := strings.SplitN(newKV, "=", 2)
 		if len(newTokens) != 2 {
 			continue
 		}
@@ -110,7 +110,7 @@ func getUpdatedVars(oldVars, newVars VariableSlice) VariableSlice {
 		found := false
 
 		for _, oldKV := range oldVars {
-			oldTokens := strings.Split(oldKV, "=")
+			oldTokens := strings.SplitN(oldKV, "=", 2)
 			if len(oldTokens) != 2 {
 				continue
 			}
@@ -168,7 +168,7 @@ func flattenEnvVarMap(varMap map[string]expand.Variable) VariableSlice {
 func MakeVariableMap(varSlice VariableSlice) VariableMap {
 	varMap := make(VariableMap)
 	for _, v := range varSlice {
-		tokens := strings.Split(v, "=")
+		tokens := strings.SplitN(v, "=", 2)
 		if len(tokens) != 2 {
 			continue
 		}