Ruby: Fix a bad join

Before
```
Evaluated relational algebra for predicate Sinatra#e09174a3::Sinatra::ErbLocalsAccessSummary#fff@22c05bb6 with tuple counts:
          212957   ~2195%    {1} r1 = JOIN _Constant#54e8b051::ConstantValue::getStringlikeValue#0#dispred#ff_Expr#6fb2af19::Expr::getConstantV__#shared WITH Expr#6fb2af19::Pair::getKey#0#dispred#ff_1#join_rhs ON FIRST 1 OUTPUT Lhs.1
        43862468   ~6045%    {2} r2 = JOIN r1 WITH Call#841c84e8::MethodCall::getMethodName#0#dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.0
        43862468   ~6581%    {2} r3 = JOIN r2 WITH AST#a6718388::AstNode::getLocation#0#dispred#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1
        43844886  ~40661%    {2} r4 = JOIN r3 WITH locations_default ON FIRST 1 OUTPUT Rhs.1, Lhs.1
           15004   ~8295%    {3} r5 = JOIN r4 WITH project#Sinatra#e09174a3::Sinatra::ErbLocalsHashSyntheticGlobal#ffff_201#join_rhs ON FIRST 1 OUTPUT Rhs.2, Lhs.1, Rhs.1
           15004   ~8890%    {3} r6 = SCAN r5 OUTPUT ("sinatra_erb_locals_access()" ++ In.0 ++ "#" ++ In.1), In.2, In.1
                             return r6
```

After
```
Evaluated relational algebra for predicate Sinatra#e09174a3::Sinatra::ErbLocalsAccessSummary#fff@f6249cga with tuple counts:
         10237       ~0%    {3} r1 = JOIN locations_default_10#join_rhs WITH project#Sinatra#e09174a3::Sinatra::ErbLocalsHashSyntheticGlobal#ffff_201#join_rhs ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Rhs.2
          4015       ~5%    {3} r2 = JOIN r1 WITH AST#a6718388::AstNode::getLocation#0#dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2
           825      ~96%    {3} r3 = JOIN r2 WITH Call#841c84e8::MethodCall::getMethodName#0#dispred#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2
           940       ~0%    {4} r4 = JOIN r3 WITH Constant#54e8b051::ConstantValue::getStringlikeValue#0#dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.0
        325402       ~0%    {4} r5 = JOIN r4 WITH Expr#6fb2af19::Expr::getConstantValue#0#dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.3
        231819  ~133147%    {3} r6 = JOIN r5 WITH Expr#6fb2af19::Pair::getKey#0#dispred#ff_1#join_rhs ON FIRST 1 OUTPUT Lhs.2, Lhs.3, Lhs.1
        231819  ~138805%    {3} r7 = SCAN r6 OUTPUT ("sinatra_erb_locals_access()" ++ In.0 ++ "#" ++ In.1), In.2, In.1
                            return r7
```

ruby/ql/lib/codeql/ruby/frameworks/Sinatra.qll

@@ -179,6 +179,12 @@ module Sinatra {
     }
   }
 
+  bindingset[local]
+  pragma[inline_late]
+  private predicate isPairKey(string local) {
+    local = any(Pair p).getKey().getConstantValue().getStringlikeValue()
+  }
+
   /**
    * A summary for accessing a local variable in an ERB template.
    * This is the second half of the modeling of the flow from the `locals`
@@ -192,7 +198,7 @@ module Sinatra {
     ErbLocalsAccessSummary() {
       this = "sinatra_erb_locals_access()" + global.getId() + "#" + local and
       local = any(MethodCall c | c.getLocation().getFile() = global.getErbFile()).getMethodName() and
-      local = any(Pair p).getKey().getConstantValue().getStringlikeValue()
+      isPairKey(local)
     }
 
     override MethodCall getACall() {