github: Make it build with Go 1.18
github: Make it build with Go 1.18

I also ran some benchmarks comparing it to Go 1.17.8:

```bash
name                                      old time/op    new time/op    delta
SiteNew/Regular_Bundle_with_image-10        5.13ms ± 7%    4.23ms ± 7%  -17.47%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10    5.12ms ± 1%    4.16ms ± 1%  -18.65%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10      17.3ms ± 2%    15.1ms ± 1%  -12.53%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10             106ms ± 1%      90ms ± 0%  -15.86%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10        34.0ms ± 1%    30.0ms ± 2%  -11.72%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10        22.7ms ± 2%    20.2ms ± 1%  -10.93%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10      16.7ms ± 0%    14.6ms ± 1%  -12.89%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10         22.2ms ± 1%    19.5ms ± 1%  -12.07%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10               7.71ms ± 0%    6.59ms ± 1%  -14.52%  (p=0.029 n=4+4)

name                                      old alloc/op   new alloc/op   delta
SiteNew/Regular_Bundle_with_image-10        2.07MB ± 0%    2.04MB ± 0%   -1.40%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10    1.85MB ± 0%    1.83MB ± 0%   -1.54%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10      11.8MB ± 0%    11.1MB ± 0%   -6.11%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10            95.8MB ± 0%    95.2MB ± 0%   -0.63%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10        27.4MB ± 0%    26.9MB ± 0%   -1.68%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10        15.2MB ± 0%    14.4MB ± 0%   -4.93%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10      8.45MB ± 0%    8.24MB ± 0%   -2.43%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10         16.5MB ± 0%    16.2MB ± 0%   -1.78%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10               3.73MB ± 0%    3.64MB ± 0%   -2.30%  (p=0.029 n=4+4)

name                                      old allocs/op  new allocs/op  delta
SiteNew/Regular_Bundle_with_image-10         25.0k ± 0%     24.1k ± 0%   -3.60%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10     25.1k ± 0%     24.1k ± 0%   -3.60%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10        237k ± 0%      234k ± 0%   -1.29%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10              365k ± 0%      363k ± 0%   -0.47%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10          284k ± 0%      279k ± 0%   -1.98%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10          252k ± 0%      249k ± 0%   -1.48%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10        117k ± 0%      115k ± 0%   -1.68%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10           184k ± 0%      181k ± 0%   -1.74%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10                49.9k ± 0%     48.5k ± 0%   -2.79%  (p=0.029 n=4+4)
```

gohugoio/hugo,gohugoio/hugo,gohugoio/hugo,anthonyfok/hugo,anthonyfok/hugo,anthonyfok/hugo