fix(health): always set 'modifiable' #40584

Problem:
When buffers are by default `nomodifiable`, such as when Nvim starts with
`-M`, the health buffer cannot be updated.

Solution:
Always set `modifiable` before modifying the buffer.

(cherry picked from commit 0f86ea5ed8)

runtime/lua/vim/health.lua

@@ -414,7 +414,6 @@ function M._check(mods, plugin_names)
       close_events = {},
     })
     vim.api.nvim_set_current_win(float_winid)
-    vim.bo[bufnr].modifiable = true
     vim.wo[float_winid].list = false
   else
     bufnr = vim.api.nvim_create_buf(true, true)
@@ -424,6 +423,7 @@ function M._check(mods, plugin_names)
     local buf_cmd = #mods > 0 and (mods .. ' sbuffer') or emptybuf and 'buffer' or 'tab sbuffer'
     vim.cmd(buf_cmd .. ' ' .. bufnr)
   end
+  vim.bo[bufnr].modifiable = true
 
   if vim.fn.bufexists('health://') == 1 then
     vim.cmd.bwipe('health://')

test/functional/plugin/health_spec.lua

@@ -140,6 +140,12 @@ describe(':checkhealth', function()
     eq(true, report:find('- custom_ft:', 1, true) ~= nil)
     eq(true, report:find('- bad_ft:', 1, true) ~= nil)
   end)
+
+  it('renders report when Nvim starts with -M', function()
+    clear { args = { '-M' } }
+    command('checkhealth lsp')
+    eq(true, curbuf_contents() ~= '')
+  end)
 end)
 
 describe('vim.health', function()