boot: lock the modeenv while reading revisions

`ubuntu-core-20-64:tests/core/system-snap-refresh` fails because
of this.

boot/boot.go

@@ -275,6 +275,9 @@ func fixedInUse(inUse bool) InUseFunc {
 // InUse returns a checker for whether a given name/revision is used in the
 // boot environment for snaps of the relevant snap type.
 func InUse(typ snap.Type, dev snap.Device) (InUseFunc, error) {
+	modeenvLock()
+	defer modeenvUnlock()
+
 	if !dev.RunMode() {
 		// ephemeral mode, block manipulations for now
 		return fixedInUse(true), nil

boot/boot_test.go

@@ -411,6 +411,47 @@ func (s *bootenvSuite) TestInUse(c *C) {
 	}
 }
 
+func (s *bootenv20Suite) TestInUseCore20(c *C) {
+	coreDev := boottest.MockUC20Device("", nil)
+	c.Assert(coreDev.HasModeenv(), Equals, true)
+	c.Assert(coreDev.IsCoreBoot(), Equals, true)
+
+	r := setupUC20Bootenv(
+		c,
+		s.bootloader,
+		&bootenv20Setup{
+			modeenv: &boot.Modeenv{
+				// base is base1
+				Base: s.base1.Filename(),
+				// no try base
+				TryBase: "",
+				// gadget is gadget1
+				Gadget: s.gadget1.Filename(),
+				// current kernels is just kern1
+				CurrentKernels: []string{s.kern1.Filename()},
+				// operating mode is run
+				Mode: "run",
+				// RecoverySystem is unset, as it should be during run mode
+				RecoverySystem: "",
+			},
+			// enabled kernel is kern1
+			kern: s.kern1,
+			// no try kernel enabled
+			tryKern: nil,
+			// kernel status is default
+			kernStatus: boot.DefaultStatus,
+		})
+	defer r()
+
+	inUse, err := boot.InUse(snap.TypeKernel, coreDev)
+	c.Check(err, IsNil)
+	c.Check(inUse(s.kern1.SnapName(), s.kern1.SnapRevision()), Equals, true)
+	c.Check(inUse(s.kern2.SnapName(), s.kern2.SnapRevision()), Equals, false)
+
+	_, err = boot.InUse(snap.TypeBase, coreDev)
+	c.Check(err, IsNil)
+}
+
 func (s *bootenvSuite) TestInUseEphemeral(c *C) {
 	coreDev := boottest.MockDevice("some-snap@install")